Vaults over leaderless processions (6)

I believe the answer is:

arches

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'vaults' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for arches that I've seen before include "Extends across" , "Under-parts of feet, sometimes fallen" , "Features of bridges or feet" , "Curves over openings" , "(Cat) stretches (back)" .)

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

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