Revolutionary given guidance provided by popular unbeliever (7)
I believe the answer is:
infidel
'unbeliever' is the definition.
(I've seen this before)
'revolutionary given guidance provided by popular' is the wordplay.
'revolutionary' says the letters should be written in reverse.
'given guidance' becomes 'led' (to lead is to give guidance).
'provided' becomes 'if' (eg 'provided it's sunny' means 'if it's sunny').
'by' is a charade indicator (letters next to each other).
'popular' becomes 'in' ('in' can mean trendy).
'led'+'if'='ledif'
'ledif' backwards is 'fidel'.
'fidel' after 'in' is 'INFIDEL'.
(Other definitions for infidel that I've seen before include "Non-believer, heretic" , "Person rejecting a specific religion" , "Pagan" , "One without religion (mainly archaic)" , "Person who has no faith, or the wrong one" .)