Wham! wholly absorbed by western piece of music (6)
I believe the answer is:
wallop
'wham' is the definition.
(synonyms)
'wholly absorbed by western piece of music' is the wordplay.
'wholly' becomes 'all' (synonyms).
'absorbed by' means one lot of letters goes inside another.
'western' becomes 'w'.
'piece of music' becomes 'op' (abbreviation for opus).
'w'+'op'='wop'
'all' going into 'wop' is 'WALLOP'.
(Other definitions for wallop that I've seen before include "Severe blow" , "Punch" , "Beat" , "Beer - hit hard" , "Strike very hard" .)