Chew like a mouse (4)
I believe the answer is:
gnaw
'chew like a mouse' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for gnaw that I've seen before include "Bite like a rodent" , "Bite and wear away" , "Chew at, work with teeth" , "Wear away with the teeth" , "Bite persistently, grind" .)