In favour of quiet support (4)
I believe the answer is:
prop
'support' is the definition.
(prop is a kind of support)
'in favour of quiet' is the wordplay.
'in favour of' becomes 'pro' (I've seen this before).
'quiet' becomes 'p' (music symbol for 'piano').
'pro'+'p'='PROP'
(Other definitions for prop that I've seen before include "Back up" , "Moveable object used on stage" , "player involved in scrum" , "Object used on the set of a play" , "Support to keep from falling" .)