Crossword Genius

Time off once a year with permission (6,5)

Ross

I believe the answer is:

annual leave

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

'year with permission' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'time off once a' is the wordplay.
I cannot really see how this works, but
'a' could be 'an' and 'an' is present in the answer.
'time off' could be 'leave' (leave is a kind of time off) and 'leave' is located in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in The Guardian.
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