If you look into the centre of the clump you should see lots of green shoots waiting for Spring. Trim back to these around about March before the shoots start to grow. Leaving the top growth on acts like a fur coat and will help protect the new growth. Trim back to 6 inches or so if the plants look too messy, then trim right back later on in March. A nice mulch of homemade compost, now, would do the plant good too. But only around the clump, not over it.