Memory-jogger about a balance (9)
I believe the answer is:
remainder
'balance' is the definition.
(I know that balance can be written as remainder)
'memory-jogger about a' is the wordplay.
'memory-jogger' becomes 'reminder' (I've seen this in another clue).
'about' means one lot of letters goes inside another.
'reminder' enclosing 'a' is 'REMAINDER'.
(Other definitions for remainder that I've seen before include "Unsold copy of book" , "The part that is left over" , "Dispose of unsold copies of book at a reduced price" , "Residue" , "book going cheap" .)