Movie advertisement (7)
I believe the answer is:
trailer
'movie advertisement' is the definition.
'trailer' can be an answer for 'advertisement' (I've seen this before). I also know that 'movie' relates to this answer.
This is the entire clue.
(Other definitions for trailer that I've seen before include "Rear-lit (anag.)" , "that's attached to back of vehicle" , "Short film plugging" , "Mobile home; preview" , "advertise film" .)