Memo about a surplus (9)
I believe the answer is:
remainder
'surplus' is the definition.
(I've seen this before)
'memo about a' is the wordplay.
'memo' becomes 'reminder' (I've seen this in another clue).
'about' is an insertion indicator.
'reminder' going around 'a' is 'REMAINDER'.
(Other definitions for remainder that I've seen before include "Residue" , "part that is still to come" , "The part that is left over" , "butt" , "Balance" .)