Republican replacing head of firm in support of monarch (5)

I believe the answer is:

royal

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'of monarch' is the definition.
(I know this)

I don't understand the remainder of the clue.

(Other definitions for royal that I've seen before include "one in palace?" , "measure of paper" , "Imperial" , "like The Grand Old Duke of York, perhaps" , "Shade of blue" .)

I've seen this clue in The Times.

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