It's used to compose pieces of music (7)
I believe the answer is:
studies
'pieces of music' is the definition.
'studies' can be an answer for 'pieces' (study is a kind of piece). I am not sure about the 'of music' bit.
'it's used to compose' is the wordplay.
'to compose' is an anagram indicator.
'its'+'used'='itsused'
'itsused' is an anagram of 'STUDIES'.
Can you help me to learn more?
(Other definitions for studies that I've seen before include "Applies the mind to learning" , "Work done to acquire learning" , "academic courses" , "Closely inspects; rooms" , "lessons" .)