Square described by Euclid? (5)

I believe the answer is:

agora

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'square' is the definition.
(I know that square can be written as agora)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for agora that I've seen before include "11 [LOLLY] to Ben-Gurion" , "A public place" , "Assembly or market place in ancient Greece" , "One hundredth of a shekel" , "Place to gather" .)

I've seen this clue in The Telegraph.

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