Chief part of the monarchy (4)

I believe the answer is:

arch

'chief' is the definition.
(synonyms)

'part of the monarchy' is the wordplay.
'part of' indicates a hidden word.
'ARCH' is hidden within 'monarchy'.

(Other definitions for arch that I've seen before include "a piece about a foot?" , "Marble, for example" , "Bridge span" , "Structural curve" , "Roguish - superior" .)

I've seen this clue in The Globe and Mail.

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