Gold transaction is nightmare (6)
I believe the answer is:
ordeal
'nightmare' is the definition.
(nightmare can mean an ordeal or difficulty)
'gold transaction' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'transaction' becomes 'deal' (a deal is a transaction or exchange).
'or'+'deal'='ORDEAL'
'is' acts as a link.
(Other definitions for ordeal that I've seen before include "A difficult and harrowing experience" , "Prolonged distressing experience" , "Long painful experience" , "Trouble" , "One may be difficult" .)