Descend from bus or train (6)
I believe the answer is:
alight
'descend from bus or train' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for alight that I've seen before include "Dismount" , "Descend, being on fire" , "Get down from, say bus" , "Step off the bus because it's on fire?" , "following action of arsonist?" .)