Chew like a beaver (4)
I believe the answer is:
'chew like a beaver' 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 "Wear away with the teeth" , "Chew steadily at" , "Nibble; cause anxiety" , "Bite persistently, grind" , "Chew persistently at, say a bone" .)