Aide-memoire about a balance (9)
I believe the answer is:
remainder
'balance' is the definition.
(I know that balance can be written as remainder)
'aide-memoire about a' is the wordplay.
'aide-memoire' becomes 'reminder' (I've seen this before).
'about' means one lot of letters goes inside another.
'reminder' placed around 'a' is 'REMAINDER'.
(Other definitions for remainder that I've seen before include "butt" , "part that is still to come" , "Balance" , "Unsold book; rest" , "Scrap" .)