Mysterious flash around plug (7)
I believe the answer is:
shadowy
'mysterious' is the definition.
(I've seen this before)
'flash around plug' is the wordplay.
'flash' becomes 'showy' (both can mean fancy or impressive).
'around' means one lot of letters goes inside another.
'plug' becomes 'ad' (Advertisement plugs products).
'showy' placed around 'ad' is 'SHADOWY'.
(Other definitions for shadowy that I've seen before include "Obscure; dim" , "hazy" , "Insubstantial" , "Obscure, dark" , "Mysterious" .)