Advert for Flash (5)
I believe the answer is:
promo
'advert' is the definition.
(I know that promo is a type of advert)
'for flash' is the wordplay.
'for' becomes 'pro' (as in being 'pro' something).
'flash' becomes 'mo' (both mean a very short period of time).
'pro'+'mo'='PROMO'
(Other definitions for promo that I've seen before include "Ad" , "Brief publicity" , "Advertising film or video" , "publicity event" , "advertisement" .)