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Private ends to serve when redoing tax return (3,2,5)

Ross

I believe the answer is:

axe to grind

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

'private ends to serve' is the definition.
I don't know anything about this answer so I can't judge whether this works.

'redoing tax return' is the wordplay.
'redoing tax' anagrams to 'AXE TO GRIND'.
But, I'm not clear how the anagram is indicated.

'when' acts as a link.

Can you help me to learn more?

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