This insect was quiet (4)
I believe the answer is:
wasp
'this insect' is the definition.
(wasp is a kind of insect)
'was quiet' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'was'+'p'='WASP'
(Other definitions for wasp that I've seen before include "colonist" , "White Anglo-Saxon Protestant" , "Insect with a formidable sting" , "Insect with a sting in its tail" , "Striped insect" .)