Note by note, old lady pockets old currency (10)
I believe the answer is:
memorandum
'note' is the definition.
(I know that memorandum is a type of note)
'note old lady pockets old currency' is the wordplay.
I cannot really understand how this works, but
'old' could be 'o' (abbreviation) and 'o' is found within the answer.
'lady' could be 'em' and 'em' is found within the answer.
'currency' could be 'rand' (I've seen this before) and 'rand' is located in the answer.
The remaining letters 'mum' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'by' acts as a link.
Can you help me to learn more?
(Other definitions for memorandum that I've seen before include "brief note" , "Written message or note" .)