Foreign banker's gold in old firm (7)
I believe the answer is:
orinoco
'foreign banker's' is the definition.
(South American river - something that banks)
'gold in old firm' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'firm' becomes 'co' (abbreviation for company).
'or'+'in'+'o'+'co'='ORINOCO'
(Other definitions for orinoco that I've seen before include "Venezuelan river; one of Wombles" , "Womble; S. Amer. river" , "River of South America" , "Colombia/Venezuela river" , "Common character" .)