Gun I stashed in seat in trailer (7)
I believe the answer is:
preview
'trailer' is the definition.
(I've seen this before)
'gun i stashed in seat' is the wordplay.
'gun' becomes 'rev' (I've seen this before).
'stashed in' means one lot of letters goes inside another.
'seat' becomes 'pew' (pew is a kind of seat).
'rev'+'i'='revi'
'revi' put within 'pew' is 'PREVIEW'.
'in' acts as a link.
(Other definitions for preview that I've seen before include "Advance showing before general release" , "early show" , "Early chance to see" , "Hint" , "See ahead" .)