Are out of a tin, but that’s the secret (6)
I believe the answer is:
Here is my best explanation:
'that's the secret' is the definition.
'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.