Are out of a tin, but that’s the secret (6)
I believe the answer is:
arcane
'that's the secret' is the definition.
(thesaurus)
'are out of a tin' is the wordplay.
'out of' means one lot of letters goes inside another.
'a tin' becomes 'can' ('can' can be a synonym of 'tin').
'are' placed around 'can' is 'ARCANE'.
'but' acts as a link.
(Other definitions for arcane that I've seen before include "Mysterious, understood by few" , "Secret, hidden" , "Mysterious - an acre (anag)" , "Recondite" , "Requiring secret or mysterious knowledge" .)