Step off the bus, because it's on fire? (6)
I believe the answer is:
alight
'step off the bus because it's on fire?' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for alight that I've seen before include "Descend, from a bus or train say" , "Leave a vehicle - burning" , "Burning; get down" , "Burning; descend" , "Step off the bus because it's on fire?" .)