Go on too long (7)
I believe the answer is:
overrun
'go on too long' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for overrun that I've seen before include "Exceed (allowed time/cost)" , "Move too fast" , "Cover - attack" , "Infest - defeat totally" , "(Programme) take too long; defeat easily" .)