Crossword Genius

I departed, heading off (3)

Ross

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" .)

I've seen this clue in The Times.
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