Complete system may be inappropriate, if not loud (4)
I believe the answer is:
unit
'complete system' is the definition.
'unit' can be an answer for 'system' (synonyms). I am not certain of the 'complete' bit.
'inappropriate if not loud' is the wordplay.
'inappropriate' becomes 'unfit' (I've seen this before).
'if not' indicates named letters should be taken away.
'loud' becomes 'f' (short for forte - 'loud' in music).
'unfit' with 'f' taken away is 'UNIT'.
'may be' is the link.
Can you help me to learn more?
(Other definitions for unit that I've seen before include "Individual thing; one" , "Part of a kit" , "- - trust" , "Individual component" , "Constituent" .)