Chew like a squirrel (4)
I believe the answer is:
gnaw
'chew like a squirrel' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for gnaw that I've seen before include "Prey on one's mind" , "Chew at, work with teeth" , "Nibble; cause anxiety" , "Nibble like a rodent" , "Chew and grind down" .)