Make nothing of repeated dirty stuff (3-3)

I believe the answer is:

poo-poo

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'repeated dirty stuff' is the definition.
I know nothing about this answer so I cannot tell whether this works.

'make nothing of' is the wordplay.
'make' becomes 'poop' (I can't explain this - if you can you should believe this answer much more).
'nothing' becomes 'o' (looks like zero - 0).
'of' becomes 'o''.
'poop'+'o'+'o'='POO-POO'

Can you help me to learn more?

I've seen this clue in The Guardian.

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