Take flight on hearing insect (4)
I believe the answer is:
flea
'insect' is the definition.
(flea is a kind of insect)
'take flight on hearing' is the wordplay.
'take flight' becomes 'flee' ('flee' can be a synonym of 'take flight').
'on hearing' shows a homophone (sound like).
'flee' sounds like 'FLEA'.
(Other definitions for flea that I've seen before include "tiny jumper" , "In one's ear, it's a stinging rebuke" , "Irritating jumper?" , "Member of a circus?" , "source of infestation" .)