Insect heard to escape (4)
I believe the answer is:
flea
'insect' is the definition.
(I know that flea is a type of insect)
'heard to escape' is the wordplay.
'heard' shows a homophone (sound like).
'to escape' becomes 'flee' (I've seen this in another clue).
'flee' is a homophone of 'FLEA'.
(Other definitions for flea that I've seen before include "source of infestation" , "Bug" , "beastie?" , "Small jumper in small circus" , "Jumping bloodsucker" .)