Wife leaves fool for friend (4)

I believe the answer is:

ally

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'friend' is the definition.
(an ally is a friend or supporter)

I don't understand the remainder of the clue.

(Other definitions for ally that I've seen before include "couple?" , "Comrade in arms" , "Accomplice" , "One in cooperation with another" , "Co-operating person or nation" .)

I've seen this clue in The Irish Times.

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