Fashionable communist collects one piece of music (7)
I believe the answer is:
introit
'piece of music' is the definition.
'introit' can be an answer for 'piece' (I have seen 'musical piece' mean 'introit' so perhaps 'piece' could also mean 'introit'). I am not sure about the 'of music' bit.
'fashionable communist collects one' is the wordplay.
'fashionable' becomes 'in' (in can mean fashionable or trendy).
'communist' becomes 'trot' (short for Trotskyist).
'collects' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'trot' going around 'i' is 'troit'.
'in'+'troit'='INTROIT'
Can you help me to learn more?
(Other definitions for introit that I've seen before include "chant" , "Short prayer" , "Choral piece" , "Something musical" , "piece of church music" .)