Go over, stifling unusual painter working (9)
I believe the answer is:
operating
'working' is the definition.
('operate' can be a synonym of 'work')
'go over stifling unusual painter' is the wordplay.
'over' indicates an anagram.
'stifling' indicates putting letters inside.
'unusual' indicates anagramming the letters.
'go' with letters rearranged gives 'og'.
'painter' anagrammed gives 'peratin'.
'og' enclosing 'peratin' is 'OPERATING'.
(Other definitions for operating that I've seen before include "Functioning" , "Directing or controlling, machinery say" , "Active" , "(Of machine) running" , "Cutting" .)