Republican's opponent in solitary, mistreated (8)
I believe the answer is:
royalist
'republican's opponent' is the definition.
'royalist' can be an answer for 'opponent' (I have seen 'opponent of Republicans' mean 'royalist' so perhaps 'opponent' could also mean 'royalist'). I am unsure of the 'republican's' bit.
'solitary mistreated' is the wordplay.
'mistreated' indicates an anagram.
'solitary' with letters rearranged gives 'ROYALIST'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for royalist that I've seen before include "Monarchist" , "rightwinger?" , "one in civil war" , "Solitary kind of person who favours monarchy" , "Supporter of monarchy" .)