Precious metal men found at a market once (5)
I believe the answer is:
agora
'a market once' is the definition.
(agoras were market spaces in ancient Greece)
'precious metal men found' is the wordplay.
'precious metal' becomes 'Ag' (Ag is the chemical symbol for silver).
'men' becomes 'OR' (Other Ranks: non-commissioned soldiers).
'found' becomes 'a' (I can't explain this - if you can you should give a lot more credence to this answer).
'ag'+'or'+'a'='AGORA'
'at' acts as a link.
Can you help me to learn more?
(Other definitions for agora that I've seen before include "meeting place" , "Marketplace in ancient Greece" , "Foreign brass" , "Place for meetings" , "(Greek) market place" .)