Some idle rodents lose ground (5)
I believe the answer is:
erode
'lose ground' is the definition.
(I've seen this in another clue)
'some idle rodents' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'ERODE' can be found hidden inside 'idle rodents'.
(Other definitions for erode that I've seen before include "Slowly remove" , "Gradually wear away" , "Eat away" , "fragment" , "Grind down" .)