Start to smoke (5,2)
I believe the answer is:
light up
'start to smoke' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for light up that I've seen before include "what floods can do" , "Ignite a cigarette; suddenly show joy" , "as one may with a cigarette" , "Start dragging in" , "Start smoking" .)