Man in toga's set about opposition (10)
I believe the answer is:
antagonism
'opposition' is the definition.
(I have seen 'Active opposition' mean 'antagonism' so perhaps 'opposition' could also mean 'antagonism')
'man in toga's set' is the wordplay.
'set' indicates anagramming the letters (instruction to set the letters).
'man'+'in'+'togas'='manintogas'
'manintogas' is an anagram of 'ANTAGONISM'.
'about' acts as a link.
(Other definitions for antagonism that I've seen before include "Active opposition" , "Active hostility" .)