The supposition is that they are lined with gold (6)
I believe the answer is:
theory
'the supposition' is the definition.
(I've seen this before)
'that they are lined with gold' is the wordplay.
'that they' becomes 'they'.
'are lined with' indicates putting letters inside (as in being filled or lined with something).
'gold' becomes 'or' ('or' is the heraldic name for gold).
'they' placed around 'or' is 'THEORY'.
'is' acts as a link.
(Other definitions for theory that I've seen before include "Explanation of a phenomenon" , "Supposition intended to explain observations" , "Supposition, hypothesis" , "Working hypothesis" , "Speculative view" .)