It's great when complainant keeps quiet (5)
I believe the answer is:
super
'it's great' is the definition.
(both can mean excellent)
'complainant keeps quiet' is the wordplay.
'complainant' becomes 'suer' (someone making a complaint in court).
'keeps' is an insertion indicator.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'suer' placed around 'p' is 'SUPER'.
'when' acts as a link.
(Other definitions for super that I've seen before include "Prefix meaning over or extremely" , "Marvellous, terrific" , "Wicked" , "Peachy" , "Excellent - small-part actor" .)