Cavalier opponents rushed and managed to score zero (10)

I believe the answer is:

roundheads

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'cavalier opponents' is the definition.
The answer and definition can be both people as well as being plural nouns.
Perhaps they are linked in a way I don't understand?

I cannot understand how the rest of the clue works.

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(Another definition for roundheads that I've seen is " Cavaliers' opponents".)

I've seen this clue in The Independent.

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