Note antique - it's valuable (4)
I believe the answer is:
gold
'valuable' is the definition.
(I know that gold is a type of precious metal)
'note antique' is the wordplay.
'note' becomes 'g' (musical note).
'antique' becomes 'old' (similar in meaning).
'g'+'old'='GOLD'
'it's' acts as a link.
(Other definitions for gold that I've seen before include "Tokens of marriage" , "first gets this?" , "Au - metal" , "Colour of archery target's centre" , "Mine produces this" .)