Silver hair ultimately concealed by a chap (6)
I believe the answer is:
argent
'silver' is the definition.
('argent' can be a synonym of 'silver')
'hair ultimately concealed by a chap' is the wordplay.
'ultimately' suggests the final letters.
'concealed by' means one lot of letters goes inside another.
'chap' becomes 'gent' (I've seen this in other clues**).
The final letter of 'hair' is 'r'.
'a'+'gent'='agent'
'r' put into 'agent' is 'ARGENT'.
(Other definitions for argent that I've seen before include "Silver on a coat of arms" , "White" , "Colour" , "Heraldic term for silver" , "(In heraldry) silver - garnet (anag)" .)