Prepare to smoke? Ray finished (5,2)
I believe the answer is:
light up
'prepare to smoke?' is the definition.
(as in lighting up a cigarette)
'ray finished' is the wordplay.
'ray' becomes 'light' (a ray of light).
'finished' becomes 'up' (eg 'time's up' means the time is finished).
'light'+'up'='LIGHT UP'
(Other definitions for light up that I've seen before include "cheer!" , "Start to smoke" , "Ignite a cigarette; suddenly show joy" , "as one may with a cigarette" , "Display joy" .)