Diplomat inattentive at the end: he's left something (7)
I believe the answer is:
legatee
'he's left something' is the definition.
'diplomat inattentive at the end' is the wordplay.
'diplomat' becomes 'legate'.
'at the end' indicates one should take the final letters.
The last letter of 'inattentive' is 'e'.
'legate'+'e'='LEGATEE'
(Other definitions for legatee that I've seen before include "One receiving something left in a will" , "One receiving legacy" , "Beneficiary of a will" , "Recipient of a bequest" , "Receiver of money from a will" .)