I’m after gold that’s fake (8)
I believe the answer is:
impostor
'fake' is the definition.
('impostor' can be a synonym of 'fake')
'i'm after gold' is the wordplay.
'after' becomes 'post' (I've seen this before).
'gold' becomes 'or' ('or' is the heraldic name for gold).
'im'+'post'+'or'='IMPOSTOR'
'that's' is the link.
(Other definitions for impostor that I've seen before include "Sit or mop (anag.)" , "Fake figure" , "One who isn't what he claims to be" , "phoney!" , "Person who pretends to be someone else" .)