TNT as in explosive flash (7)
I believe the answer is:
instant
'flash' is the definition.
(synonyms)
'tnt as in explosive' is the wordplay.
'explosive' is an anagram indicator.
'tnt'+'as'+'in'='tntasin'
'tntasin' is an anagram of 'INSTANT'.
(Other definitions for instant that I've seen before include "Sort of coffee" , "Occurring immediately, like some fast food" , "Trice" , "Mo" , "Immediate; moment in time" .)