You can use a selective herbicide which will kill broad leaved plants but not grasses. The grass won't be dead, but just very dry. The roots will still regrow. After applying the herbicide you may need to re-seed a little fresh grass seed if there are bare patches, and give a good general feed to 'green up' the grass.
However, always remember that clover gives you a nice green lawn even in drought conditions, while grasses will go brown.
The clover won't parasite the grass, in fact the root nodules create nitrogen which will feed the ground and be released whent the clover is killed. All that's happening is it is crowding out the grass and taking moisture and light away.

Bertifox's explanation is accurate - the clover's been there for ages, but you haven't noticed it, and you're only noticing it now because of the drought - the grass has gone brown or yellow and the only green thing out there is the clover - because of its nitrogen fixing roots, it stays green. When the rains come and the grass greens up again, it will be much less noticeable. There used to be a product available as a lawn treatment that did actually kill clover 20 years ago, but it was withdrawn because it was dangerous in the environment - I have never yet found any suitable substitute for it, so I'm not convinced any lawn treatments contain a herbicide which will work on clover. If you find one that does the job, I'd be pleased to hear about it, lol

Bamboo and Bertiefox,you are both far more experienced gardeners than I am,but
we are about to use GRAZON,which I think was previously known as BROADSHOT.
I understand it will kill everything but the actual grass.
It is used on paddocks and the horses are allowed back after seven days.
It is not normally found in garden centres and is quite expensive.It can be found on the web if you ask google for' Grazon'.
We are expecting to find lots of bare patches when all the weeds are dead but hopefully will be able to re- seed the patches and have a better lawn next year.I do not know if this is considered bad for the environment,but it works for the horses and recovers a lot of ground ,providing more grazing,so hopefully it will work on our
lawns.

If you want to use Grazon, which contains 2 4-D, I suggest you read this first.
..
"Grazon is a restricted-use pesticide made by Dow AgroSciences. It is used to control broadleaf annual and perennial weeds and certain woody species. According to the label, its active ingredients are picloram: 4-amino-3,5,6-tricholorpicolinic acid, triisopropanolamine salt (10.2%), and 2,4-dicholorphenoxyacetic acid, triisopropanolamine salt (39.6%). If you use Grazon, follow the manufacturer's safety recommendations for the safety of both yourself and other plants in your garden.

Personal Dangers
Grazon is corrosive, can cause irreversible eye damage, and is harmful if swallowed or inhaled. When using, do not get the herbicide in your eyes or on your clothing. Do not breathe the spray mist. Wear protective clothing, chemical-resistant gloves and protective eyewear. Grazon can irritate the skin, cause serious damage to eyes, and is harmful if swallowed.

Environmental Hazards
Grazon is toxic to some plants even at very low concentrations. Do not use it on windy days, as the spray may drift to plants you don't want to injure. The manufacturer warns that you should not apply Grazon directly to water, areas where surface water is present or to intertidal areas below the mean high water mark. When cleaning your equipment, do not contaminate water used for domestic or irrigation purposes. The ingredient picloram can leach through soil into ground water. If you use the chemical in areas where soils are permeable, you may contaminate the ground water, according to the manufacturer. Grazon is very toxic to aquatic organisms, and it can cause long-term adverse effects in the aquatic environment, Dow-Agro warns.

Physical and Chemical Hazards
Grazon should not be stored near heat or open flame. It should not be shipped or stored with food, animal feed, drugs or clothing. Never apply Grazon through an irrigation system. Do not apply if the drift will come in contact with people.

Animal Hazards
Lactating dairy animals should not be allowed to graze treated areas within seven days of application. Hay should not be harvested within 30 days of application. Meat animals should not forage on treated areas at least three days before slaughter. Livestock that have grazed on treated areas should not be moved to broadleaf crop areas without first having seven days of grazing on untreated pasture. The animals' urine may contain enough picloram, the manufacturer warns, to injure sensitive broadleaf plants. Do not compost or mulch plants with grass or hay from treated areas, or with manure from animals that have foraged on treated areas.

Read more: What Are the Dangers of Grazon Herbicide? | Garden Guides http://www.gardenguides.com/88506-dangers-grazon-herbicide.html#ixzz0tNrNi9aE