Work out what's behind it all (6)

I believe the answer is:

reason

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'work out' is the definition.
(thesaurus)

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

(Other definitions for reason that I've seen before include "Cause to perform" , "Motivation or cause" , "logical argument" , "Cause, explanation" , "Deduce" .)

I've seen this clue in the Evening Standard.

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