Stand in as secret revolutionary (10)
I believe the answer is:
resistance
'stand' is the definition.
(thesaurus)
'in as secret revolutionary' is the wordplay.
'revolutionary' indicates anagramming the letters.
'in'+'as'+'secret'='inassecret'
'inassecret' anagrammed gives 'RESISTANCE'.
(Other definitions for resistance that I've seen before include "Withstanding" , "French fighting group" , "Rebels" , "Freedom fighters - it's measured in ohms" , "immunity" .)