Gnawing mammal (6)
I believe the answer is:
rodent
'gnawing mammal' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rodent that I've seen before include "Eg, mouse, squirrel" , "Mouse, squirrel or rat" , "A rat, for instance" , "Eg, mouse or rat" , "Rat perhaps" .)