Reason for a solder to be in some locality (5)

I believe the answer is:

logic

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'reason' is the definition.
(logic is using reason)

I can't explain the remainder of the clue.

(Other definitions for logic that I've seen before include "Good reason" , "Aristotelian theories" , "The science of reasoning correctly" , "System of reasoning" , "4-ing" .)

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