Memo to take a rest (9)
I believe the answer is:
remainder
'rest' is the definition.
(synonyms)
'memo to take a' is the wordplay.
'memo' becomes 'reminder' (I've seen this in another clue).
'to take' indicates putting letters inside (inserted letters are taken in).
'reminder' enclosing 'a' is 'REMAINDER'.
(Other definitions for remainder that I've seen before include "Surplus" , "Left-over" , "Unsold copy of book" , "Balance" , "set of unsold books" .)