I have to admit the words practice and practise gave me a lot of grief. I know the obvious usage but when I saw 'practice makes perfect' it didn't feel right, even though it is. Now I'm a tad better at working out those situations where I still struggle. For me this is like a fun puzzle.
OK. Then. The answer is a practice piece. Why?
You are writing the piece to be used as part of practice and thus a practice piece. You could practise using a practice piece, but you write a practice piece. The piece is for practice which is the regular activity and thus it is a practice piece. You could say I'm going to practise my writing by writing a practice piece, but that's not saying the same thing.
Sometimes we can get confused when we shorten our sentences. It is possible the person's question being quite short may not be clear as the what they really want, but all we can do is to respond to the question as it is asked. A better question would probably be, what do you mean?
Creator of the preferred Australian English spelling dictionary.