Note written about a surplus (9)
I believe the answer is:
remainder
'a surplus' is the definition.
(I've seen this before)
'note written about' is the wordplay.
I cannot really see how this works, but
'note' could be 're' (as in do-re-mi) and 're' is located in the answer.
'about' could be 'a' (abbreviation in Chambers) and 'a' is present in the answer.
The remaining letters 'minder' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for remainder that I've seen before include "Leftovers" , "It's left over after division" , "Left-over" , "set of unsold books" , "Balance" .)