Mysterious flash enveloping trailer (7)
I believe the answer is:
shadowy
'mysterious' is the definition.
(I've seen this before)
'flash enveloping trailer' is the wordplay.
'flash' becomes 'showy' (both can mean fancy or impressive).
'enveloping' is an insertion indicator.
'trailer' becomes 'ad' (short for advert).
'showy' enclosing 'ad' is 'SHADOWY'.
(Other definitions for shadowy that I've seen before include "Obscure, dark" , "Insubstantial" , "Obscure; dim" , "Why soda? (anag)" , "Mysterious" .)