Crossword Genius

Twice old flyer pens single: a 60s earworm (2-2-2,2-2-2)

Ross

I believe the answer is:

ob-la-di ob-la-da

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'60s earworm' is the definition.
I know nothing about this answer so I cannot tell whether this works.

'twice old flyer pens single a' is the wordplay.
I cannot really understand how this works, but
'old' could be 'o' (abbreviation) and 'o' is present in the answer.
'flyer' could be 'ad' (advertisement) and 'ad' is present in the answer.
'single' could be 'i' (one) and 'i' is found within the answer.
'a' is present in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

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