Seat about six inside trailer (7)
I believe the answer is:
preview
'trailer' is the definition.
(I know that trailer can be written as preview)
'seat about six' is the wordplay.
I cannot really see how this works, but
'about' could be 're' ('re' means regarding or about) and 're' is found in the answer.
'six' could be 'vi' (Roman numerals) and 'vi' is found within the answer.
'seat' could be 'pew' (pew is a kind of seat) and 'pew' is found within the leftover letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'inside' acts as a link.
(Other definitions for preview that I've seen before include "Foretaste" , "Film trailer" , "early show" , "Preliminary showing" , "Advance showing before general release" .)