Crossword Genius

Time for moon shape with biro twist by headmaster (3,2)

Ross

I believe the answer is:

one pm

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

'moon shape with biro twist by headmaster' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'time' is the wordplay.
I cannot quite understand how this works, but
'time' could be 'one' (I've seen this before) and 'one' is found within the answer.
This may be the basis of the clue (or it may be nonsense).

'for' is the link.

Can you help me to learn more?

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