Revolutionary sects are in opposition (10)
I believe the answer is:
resistance
'opposition' is the definition.
('resistance' can be a synonym of 'opposition')
'revolutionary sects are in' is the wordplay.
'revolutionary' indicates an anagram.
'sects'+'are'+'in'='sectsarein'
'sectsarein' anagrammed gives 'RESISTANCE'.
(Other definitions for resistance that I've seen before include "Violent opposition" , "immunity" , "Withstanding" , "underground movement" , "Rebellious group" .)