Voice disapproval when volume reduced (3)
I believe the answer is:
boo
'voice disapproval' is the definition.
(I've seen this before)
'volume reduced' is the wordplay.
'volume' becomes 'book' (book is a kind of volume).
'reduced' means to remove the last letter.
'book' with its last letter taken off is 'BOO'.
'when' is the link.
(Other definitions for boo that I've seen before include "Exclamation to startle" , "Poor reception" , "causing surprise!" , "get off!" , "Make one's dislike heard" .)