Nibble or bite like a rodent (4)
I believe the answer is:
gnaw
This clue is a double definition.
'nibble' is the first definition.
(I've seen this in another clue)
'bite like a rodent' is the second definition.
(I've seen this before)
(Other definitions for gnaw that I've seen before include "Wear away with the teeth" , "Bite like a rodent" , "Bite persistently, grind" , "Chew away at" , "Nibble; cause anxiety" .)