The reasoning applied to certain puzzles (5)

I believe the answer is:

logic

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'the reasoning' is the definition.
(I've seen this before)

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

(Other definitions for logic that I've seen before include "The science of reasoning from cause to effect" , "Good reason" , "Reasoned judgement" , "Aristotelian theories" , "Branch of philosophy that analyses inference" .)

I've seen this clue in the Evening Standard.

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