Monetary system introduced by firm when old (7)
I believe the answer is:
coinage
'monetary system' is the definition.
(coinage is a kind of monetary system)
'introduced by firm when old' is the wordplay.
I cannot quite see how this works, but
'firm' could be 'co' (a firm is a company) and 'co' is located in the answer.
'when' could be 'e' (I've seen this in other clues) and 'e' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for coinage that I've seen before include "for the new term" , "in notes? No" , "Some ready" , "Royal Mint's output" , "Metal currency" .)