Underlying reason (9)

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4,5

rationale

rootcause

'underlying reason' is the definition.
(I've seen this in another clue)

This is the entire clue.

(Other definitions for rationale that I've seen before include "Logic, reasoning" , "explanation" , "grounds?" , "Underlying principle" , "Why regulate" .)

I've seen this clue in the Universal, The Sun, the USA Today and The New York Times.

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