Note it's old currency (4)
I believe the answer is:
mark
I believe this is a double definition.
'note' is the first definition.
('mark' can be a synonym of 'note')
'old currency' is the second definition.
(I've seen this before)
'it's' acts as a link.
(Other definitions for mark that I've seen before include "Former German currency" , "Smudge" , "follower of Jesus" , "Former German coin" , "Old money" .)