Go on too long? Left work (7)
I believe the answer is:
overrun
'go on too long?' is the definition.
(I've seen this before)
'left work' is the wordplay.
'left' becomes 'over' (I've seen this before).
'work' becomes 'run' (both can mean to operate something).
'over'+'run'='OVERRUN'
(Other definitions for overrun that I've seen before include "Go beyond (time/budget)" , "Exceed time allowed, or spread across and injure (7)" , "Flood" , "(Programme) take too long; defeat easily" , "Stretch too far" .)