Square revolutionary fastening clasps (4)

I believe the answer is:

nine

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'square' is the definition.
(nine is a square number)

I can't explain the remainder of the clue.

(Other definitions for nine that I've seen before include "One more than fourteen across" , "Number of players in a baseball fielding team" , "Number of Muses and days in a Novena" , "Number for a club" , "One's married partner" .)

I've seen this clue in The Telegraph.

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