Leaf-chewing insect (4)
I believe the answer is:
flea
'insect' is the definition.
(flea is a kind of insect)
'leaf-chewing' is the wordplay.
'chewing' is an anagram indicator.
'leaf' anagrammed gives 'FLEA'.
(Other definitions for flea that I've seen before include "Small jumper in small circus" , "Tiny parasite" , "Jumping bloodsucker" , "Jumping, biting insect" , "tiny jumper" .)