Is going to leave (4)
I believe the answer is:
will
I believe this is a double definition.
'is going to' is the first definition.
(John is going to eat/John will eat)
'leave' is the second definition.
('will' can be a synonym of 'leave')
(Other definitions for will that I've seen before include "Fixed intention" , "Shakespeare < 's intention to leave something" , "Volition" , "self-control" , "Last testament" .)