I departed, heading off (3)
I believe the answer is:
one
'i' is the definition.
('one' can be a synonym of 'i')
'departed heading off' is the wordplay.
'departed' becomes 'gone' ('gone' can be a synonym of 'departed').
'heading off' means to remove the first letter.
'gone' with its first letter taken off is 'ONE'.
(Other definitions for one that I've seen before include "Ace" , "1" , "Smallest whole number" , "Single and undivided" , "time" .)