Be dreamily infatuated about a vehicle in light biscuit (8)
I believe the answer is:
macaroon
'light biscuit' is the definition.
(I've seen this before)
'be dreamily infatuated about a vehicle' is the wordplay.
'be dreamily infatuated' becomes 'moon' (as in mooning over somebody you fancy).
'about' means one lot of letters goes inside another.
'vehicle' becomes 'car' (car is a kind of vehicle).
'a'+'car'='acar'
'moon' going around 'acar' is 'MACAROON'.
'in' is the link.
(Other definitions for macaroon that I've seen before include "Something nutty to enjoy" , "Sweet bit of food" , "Type of biscuit" , "Biscuit of egg-white and almonds or coconut" , "Almond or coconut biscuit" .)