Source of genuinely ancient metal (4)
I believe the answer is:
gold
'metal' is the definition.
(gold is a kind of metal)
'source of genuinely ancient' is the wordplay.
'source of' indicates taking the first letters.
'ancient' becomes 'old' ('old' can be similar in meaning to 'ancient').
The initial letter of 'genuinely' is 'g'.
'g'+'old'='GOLD'
(Other definitions for gold that I've seen before include "gift from a king" , "bull's-eye" , "Wealth; first prize" , "Valuable metal" , "Deep yellow" .)