Republican sees I can't organise opposition (10)
I believe the answer is:
resistance
'opposition' is the definition.
('resistance' can be a synonym of 'opposition')
'republican sees i can't organise' is the wordplay.
'republican' becomes 'r' (abbreviation).
'organise' is an anagram indicator (I've seen 'organising' mean this).
'sees'+'i'+'cant'='seesicant'
'seesicant' with letters rearranged gives 'esistance'.
'r'+'esistance'='RESISTANCE'
(Other definitions for resistance that I've seen before include "fighters in wartime" , "Rebels" , "immunity" , "Freedom fighters - it's measured in ohms" , "underground movement" .)