Escape a zipper that isn't on (3,3)

I believe the answer is:

fly off

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

'isn't on' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'escape a zipper' is the wordplay.
'escape' becomes 'fly' (I've seen this in another clue).
'a zipper' becomes 'off' (I can't explain this - if you can you should believe this answer much more).
'fly'+'off'='FLY OFF'

'that' is the link.

Can you help me to learn more?

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

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