Proper-sounding form of service (4)
I believe the answer is:
rite
'form of service' is the definition.
(I've seen this before)
'proper-sounding' is the wordplay.
'proper' becomes 'right' ('right' can be a synonym of 'proper').
'sounding' indicates a 'sounds like' (homophone) clue.
'right' sounds like 'RITE'.
(Other definitions for rite that I've seen before include "form of service" , "Tire of ceremony" , "Mystery - ceremony" , "Uncomplaining person" , "Ceremonial act" .)