Leave backer trapped with no back up (6)

I believe the answer is:

depart

I'm a little stuck... Click here to teach me more about this clue!

'leave' is the definition.
(I know that leave can be written as depart)

I cannot understand how the remainder of the clue works.

(Other definitions for depart that I've seen before include "Take one's leave, go" , "Start out" , "Stray" , "Set out" , "make tracks" .)

I've seen this clue in the Sydney Morning Herald.

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