Roundhead’s opponent, offhand (8)
I believe the answer is:
cavalier
This clue might be a double definition.
'roundhead's opponent' is the first definition.
The definition and answer can be both people as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'offhand' is the second definition.
(I've seen this before)
Can you help me to learn more?
(Other definitions for cavalier that I've seen before include "'Thoughtless, offhand (8)'" , "Roundhead's foe" , "Offhand - Charles I supporter" , "Herrick and Lovelace were such poets" , "... King Charles, canine breed" .)