Crossword Genius

Central part of a masonry arch (8)

Ross

I believe the answer is:

keystone

(Other definitions for keystone that I've seen before include "Piece at the top of a masonry arch" , "Important piece at apex of arch" , "Kop" , "What tops an arch" , "everything may depend on it" .)

I've seen this clue in the Sydney Morning Herald.
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