Heard insects in leaves (5)
I believe the answer is:
flees
'leaves' is the definition.
(fleeing is a kind of leaving)
'heard insects' is the wordplay.
'heard' indicates a 'sounds like' (homophone) clue.
'insects' becomes 'fleas' (flea is a kind of insect).
'fleas' is a homophone of 'FLEES'.
'in' acts as a link.
(Other definitions for flees that I've seen before include "flies" , "Runs away" .)