Crossword Genius

"Yesterday" he wrote, accurately, "with slip-up forgotten, Mum finally relaxed" (4,9)

Ross

I believe the answer is:

paul mccartney

'yesterday he wrote' is the definition.
(Paul McCartney wrote the song Yesterday)

I can't explain the remainder of the clue.

(Another definition for paul mccartney that I've seen is " Singer".)

I've seen this clue in The Independent.
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