Gentleman I ditched for a fuddy-duddy (6)

I believe the answer is:

square

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'fuddy-duddy' is the definition.
(thesaurus)

I don't understand how the remainder of the clue works.

(Other definitions for square that I've seen before include "Old-fashioned - fix" , "area of town" , "Old-fashioned person" , "quits" , "Bribe" .)

I've seen this clue in The Times.

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