A measure of antiquity (5)

I believe the answer is:

cubit

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'a measure' is the definition.
(cubit is a kind of measure)

I don't understand the remainder of the clue.

(Other definitions for cubit that I've seen before include "Ancient linear measure" , "about eighteen inches" , "distance" , "Ancient measure about the length of an arm" , "Measurement based on the length of the forearm" .)

I've seen this clue in the Evening Standard.

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