Fight is tense, having fixed boxing ring (3-2)

I believe the answer is:

set-to

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'fight' is the definition.
(I know that set-to is a type of fight)

I can't explain the remainder of the clue.

(Other definitions for set-to that I've seen before include "Fight, argument" , "Brief fight" , "brush with violence?" , "Scrap" , "Altercation" .)

I've seen this clue in The Guardian.

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