Complain when buzzer is on loud (4)
I believe the answer is:
beef
'complain' is the definition.
(beefing is a kind of complaining)
'buzzer is on loud' is the wordplay.
'buzzer' becomes 'bee' (I've seen this before).
'is on' is a charade indicator (letters next to each other).
'loud' becomes 'f' (A musical term).
'bee'+'f'='BEEF'
'when' acts as a link.
(Other definitions for beef that I've seen before include "Grumble - muscle" , "See 24" , "Complaint; strength" , "Complain; meat" , "Small errand" .)