Overlook setter's memorandum (7)
I believe the answer is:
missive
'memorandum' is the definition.
(thesaurus)
'overlook setter's' is the wordplay.
'overlook' becomes 'miss' ('miss' can be a synonym of 'overlook').
'setter's' becomes 'ive' (I've seen this in other clues).
'miss'+'ive'='MISSIVE'
(Other definitions for missive that I've seen before include "message" , "intelligence in this?" , "Letter" .)