Silver dollar ultimately pocketed by spy (6)
I believe the answer is:
argent
'silver' is the definition.
('argent' can be a synonym of 'silver')
'dollar ultimately pocketed by spy' is the wordplay.
'ultimately' suggests the final letters.
'pocketed by' indicates putting letters inside.
'spy' becomes 'agent' (I've seen this before).
The last letter of 'dollar' is 'r'.
'r' going within 'agent' is 'ARGENT'.
(Other definitions for argent that I've seen before include "Heraldic silver in garnet" , "Silver in 1 Down" , "(In heraldry) silver - garnet (anag)" , "White" , "Silver on a coat of arms" .)