Film preview in caravan? (7)
I believe the answer is:
trailer
This clue is a double definition.
'film preview' is the first definition.
(I've seen this before)
'caravan?' is the second definition.
(caravan is a kind of trailer)
'in' is the link.
(Other definitions for trailer that I've seen before include "Towed vehicle; advertisement" , "TV ad" , "Short scenes from a coming film" , "one left behind" , "One comes before the big film" .)