Dismount in flames (6)
I believe the answer is:
alight
This clue is a double definition.
'dismount' is the first definition.
(alight can mean to get off or dismount)
'in flames' is the second definition.
(alight can mean on fire or ablaze)
(Other definitions for alight that I've seen before include "Ignited; get off" , "Get down from, say bus" , "Get out of train" , "Disembark; on fire" , "Small restaurant" .)