In old gold can be unyielding (10)
I believe the answer is:
inexorable
'unyielding' is the definition.
(thesaurus)
'in old gold can' is the wordplay.
'old' becomes 'ex'.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'can' becomes 'able' (I've seen this in other clues**).
'in'+'ex'+'or'+'able'='INEXORABLE'
'be' is the link.
(Other definitions for inexorable that I've seen before include "Relentless" , "Unrelenting" , "Inescapable" , "Impossible to stop" , "Unstoppable" .)