Looks like clover so a broad-leaf weedkiller should get rid of it. Mind, there is not much grass there so it will be pretty bald afterwards,

If this isn't too large a lawn, perhaps think about a total weed killer and start with completely new lawn. Turf or seed, this is a good time to renew.

Doesn't look to be a huge area? If it's just a small patch you could cover it with cardboard, weight it down & then remove, aerate the soil & re-seed in Spring.
Worth considering it's much better for bees than grass though!

certainly one of the clover family. has it had flowers and if so what colour. there is a small yellow one called black medic leaves about 5mm in length.