Work out why (6)

I believe the answer is:

reason

This clue is a double definition.

'work out' is the first definition.
(thesaurus)

'why' is the second definition.
(thesaurus)

(Other definitions for reason that I've seen before include "'Motivation, grounds (6)'" , "Logic of senora" , "Justification needed" , "Cause to perform" , "Logic, sense" .)

I've seen this clue in The Telegraph.

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