Notice about old currency (6)
I believe the answer is:
remark
'notice' is the definition.
(remark is a kind of notice)
'about old currency' is the wordplay.
'about' becomes 're' (regarding).
'old currency' becomes 'mark' (I've seen this before).
're'+'mark'='REMARK'
(Other definitions for remark that I've seen before include "(E.g. Virgil's) pastoral poems" , "Make mention of" , "Notice or comment on" , "Say as a comment" , "Written or spoken comment" .)