Broker handling king's silver (6)
I believe the answer is:
argent
'silver' is the definition.
('argent' can be a synonym of 'silver')
'broker handling king's' is the wordplay.
'broker' becomes 'agent' ('agent' can be a synonym of 'broker').
'handling' means one lot of letters goes inside another.
'king' becomes 'r' (r is abbreviation for rex).
'agent' enclosing 'r' is 'ARGENT'.
(Other definitions for argent that I've seen before include "White" , "Silver on a coat of arms" , "(In heraldry) silver - garnet (anag)" , "Colour" , "Silver in 1 Down" .)