Get off - it's on fire (6)
I believe the answer is:
alight
I believe this clue is a double definition.
'get off' is the first definition.
(I've seen this before)
'on fire' is the second definition.
(synonyms)
'it's' acts as a link.
(Other definitions for alight that I've seen before include "Land; burning" , "Descend from vehicle" , "in flames" , "Brilliant" , "Land, come down" .)