Instrument lined with lead to reinforce old metal (6)
I believe the answer is:
argent
'old metal' is the definition.
(historical name for silver)
'instrument lined with lead to' is the wordplay.
'instrument' becomes 'agent' (I've seen this before).
'lined with' means one lot of letters goes inside another (as in being filled or lined with something).
'lead to' becomes 'r' (I can't explain this - if you can you should believe this answer much more).
'agent' placed around 'r' is 'ARGENT'.
'reinforce' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for argent that I've seen before include "Silver on a coat of arms" , "Heraldic silver in garnet" , "(In heraldry) silver - garnet (anag)" , "Silver in 1 Down" , "Colour" .)