Doesn't sit idly by (4)
I believe the answer is:
acts
'doesn't sit idly by' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for acts that I've seen before include "appears (eight in a series may be found)" , "Performs on stage" , "exerts influence" , "Behaves or plays" , "Goes" .)