Mounted diamonds and gold bar (3)
I believe the answer is:
rod
'bar' is the definition.
(both can mean a pole or stick)
'mounted diamonds and gold' is the wordplay.
'mounted' is a reversal indicator (in a down clue, letters mounted up).
'diamonds' becomes 'd' (abbreviation in bridge say).
'and' means one lot of letters go next to another.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'd'+'or'='dor'
'dor' written backwards gives 'ROD'.
(Other definitions for rod that I've seen before include "Long bar or pole" , "Sceptre" , "Angler's cane" , "Straight, slender stick" , "Thin straight bar" .)