Crossword Genius

The secret of Skinny's biography - nothing's replaced by gibberish, essentially (7)

Ross

I believe the answer is:

mystery

'the secret' is the definition.
(thesaurus)

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

(Other definitions for mystery that I've seen before include "Secret doctrine" , "A thing hard to explain" , "Puzzle" , "thriller" , "Mediaeval trade" .)

I've seen this clue in The Independent.
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