Go off to take the vehicle back β a motor vehicle (7)
I believe the answer is:
tractor
'a motor vehicle' is the definition.
(tractor is a kind of motor vehicle)
'go off to take the vehicle back' is the wordplay.
'go off' becomes 'rot' (I've seen this before).
'to take' means one lot of letters go next to another.
'the vehicle' becomes 'cart' (cart is a kind of vehicle).
'back' shows that the letters should be reversed in order.
'rot'+'cart'='rotcart'
'rotcart' reversed gives 'TRACTOR'.
(Other definitions for tractor that I've seen before include "Type of truck with a a cab but no body" , "Tower" , "Agricultural vehicle" , "Farming vehicle" , "Agricultural machine" .)