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