It's about old currency notes (7)
I believe the answer is:
remarks
'notes' is the definition.
('remark' can be a synonym of 'note')
'it's about old currency' is the wordplay.
'it's about' becomes 're' (regarding).
'old currency' becomes 'marks' (former currency of Germany**).
're'+'marks'='REMARKS'
(Other definitions for remarks that I've seen before include "Comments, observations" , "See 14" .)