River making gold in old firm (7)
I believe the answer is:
orinoco
'river' is the definition.
(I know that Orinoco is a river)
'gold in old firm' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'old' becomes 'o' (abbreviation).
'firm' becomes 'co' (a firm is a company).
'or'+'in'+'o'+'co'='ORINOCO'
'making' acts as a link.
(Other definitions for orinoco that I've seen before include "River of South America" , "O, I croon about the river" , "Great river in the north of S America, flows through Venezuela" , "Great river flowing through Venezuela to Atlantic" , "banker" .)