Some fierce rodents grind away (5)
I believe the answer is:
erode
'grind away' is the definition.
(I've seen this in another clue)
'some fierce rodents' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'ERODE' is hidden in the letters of 'fierce rodents'.
(Other definitions for erode that I've seen before include "Corrode" , "Gradually destroy" , "Gradually wear away" , "'Deteriorate, wear away (5)'" , "go to pieces!" .)