Anvil is iron on the outside, and every other part of course (5)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

I(ro)N, + C(o)U(r)S(e). The incus is a bone in the middle ear that resembles an anvil. If you squint. Possibly. A little. Tricky if you don’t know the word, but at least you do get three crossers.