Arch-supporter of comic policemen (8)

I believe the answer is:

keystone

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'arch' is the definition.
(thesaurus)

I don't understand the rest of the clue.

(Other definitions for keystone that I've seen before include "Central principle of a policy" , "Something on which all else depends" , "Central part of a system" , "Piece at the top of a masonry arch" , "Kop" .)

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

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