Oliver, Parliamentarian (9)

I believe the answer is:

roundhead

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'parliamentarian' is the definition.
(I know that parliamentarian can be written as Roundhead)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for roundhead that I've seen before include "An enemy of Charles" , "Supporter of Parliament during the English Civil Wars" , "Parliamentarian (hist.)" , "antimonarchist" , "Old soldier" .)

I've seen this clue in the Evening Standard.

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