Popular exploit, without a doubt (6)
I believe the answer is:
indeed
'without a doubt' is the definition.
(I've seen this in another clue)
'popular exploit' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'exploit' becomes 'deed' (synonyms).
'in'+'deed'='INDEED'
(Other definitions for indeed that I've seen before include "Really, actually" , "To be sure" , "Very much so" , "Actually -- denied (anag)" , "Surely" .)