Towed vehicle in film advertisement (7)
I believe the answer is:
trailer
I believe this is a double definition.
'towed vehicle' is the first definition.
(I've seen this before)
'film advertisement' is the second definition.
(I've seen this in another clue)
'in' acts as a link.
(Other definitions for trailer that I've seen before include "Advert; one behind car" , "advertise film" , "Extract for advance publicity" , "Picture" , "It can be towed or be seen in a cinema" .)