Vehicle to bring back horse beyond marsh (8)
I believe the answer is:
toboggan
'vehicle' is the definition.
(toboggan is a kind of vehicle)
'to bring back horse beyond marsh' is the wordplay.
'bring back' shows that the letters should be reversed in order.
'horse' becomes 'nag' (nag is a kind of horse**).
'beyond' is a charade indicator (letters next to each other).
'marsh' becomes 'bog' (I've seen this before).
'nag' reversed gives 'gan'.
'gan' after 'bog' is 'boggan'.
'to'+'boggan'='TOBOGGAN'
(Other definitions for toboggan that I've seen before include "Sled for sliding on snow" , "Sled with boards curved up in front" , "Slider" , "Light sledge for sliding downhill" , "Travel in it" .)