Step off when burning (6)
I believe the answer is:
alight
I believe this clue is a double definition.
'step off' is the first definition.
(I've seen this in another clue)
'burning' is the second definition.
(set alight)
'when' acts as a link.
(Other definitions for alight that I've seen before include "Come to rest, settle" , "Leave a vehicle - burning" , "Descend, being on fire" , "Settle; on fire" , "in flames" .)