Sorry, that is, to leave Sadie (3)
I believe the answer is:
sad
'sorry' is the definition.
(I know that sorry can be written as sad)
'that is to leave sadie' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'to leave' suggests deleting specific letters.
'sadie' with 'ie' taken away is 'SAD'.
(Other definitions for sad that I've seen before include "Deplorable" , "Unhappy or melancholy" , "Mournful and depressed" , "Miserable" , "Sorrowful, unhappy" .)