Insect beat a hasty retreat, we hear (4)
I believe the answer is:
flea
'insect' is the definition.
(I know that flea is a type of insect)
'beat a hasty retreat we hear' is the wordplay.
'beat a hasty retreat' becomes 'flee' (I've seen this in another clue).
'we hear' indicates a 'sounds like' (homophone) clue.
'flee' is a homophone of 'FLEA'.
(Other definitions for flea that I've seen before include "little pest" , "irritant type" , "In your ear, this insect is a rebuke" , "Jumping, blood-sucking insect" , "Tiny parasite" .)