Vehicle in ambush (3)
I believe the answer is:
bus
'vehicle' is the definition.
(bus is a kind of vehicle)
'in ambush' is the wordplay.
'in' says the answer is hidden in the clue.
'BUS' is hidden in the letters of 'ambush'.
(Other definitions for bus that I've seen before include "Road vehicle able to carry many passengers" , "Public service vehicle" , "eg Double-decker" , "Of projectiles; (go) mad (slang)" , "Mode of public transport" .)