Some of the rodents gnaw away (5)
I believe the answer is:
erode
'gnaw away' is the definition.
(thesaurus)
'some of the rodents' is the wordplay.
'some of' indicates a hidden word.
'ERODE' is hidden within 'the rodents'.
(Other definitions for erode that I've seen before include "Gradually wear away" , "Gradually destroy" , "Abrade" , "go to pieces!" , "'Deteriorate, wear away (5)'" .)