Vehicle, one held in reserve, coming up (3)
I believe the answer is:
bus
'vehicle' is the definition.
(bus is a kind of vehicle)
'one held in reserve coming up' is the wordplay.
'one held in reserve' becomes 'sub' (as in a substitute in sport).
'coming up' shows that the letters should be reversed in order.
'sub' back-to-front is 'BUS'.
(Other definitions for bus that I've seen before include "Mode of public transport" , "Large passenger vehicle - small business!" , "Public transport vehicle" , "People carrier" , "Means of public transport" .)