Two vehicles including a trailer (7)
I believe the answer is:
caravan
'trailer' is the definition.
(caravan is a kind of trailer)
'two vehicles including a' is the wordplay.
'two vehicles' means two replacements for 'vehicle'.
'vehicle' becomes 'car' (car is a kind of vehicle).
'vehicle' becomes 'van' (type of vehicle).
'including' is an insertion indicator.
'car'+'van'='carvan'
'carvan' going around 'a' is 'CARAVAN'.
(Other definitions for caravan that I've seen before include "Towed accommodation" , "Group of desert voyagers" , "Mobile home" , "Towable home" , "Towed vehicle" .)