12 across
Patsy’s
date
of
birth?
(9)
Patsy’s date of birth? (9)
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The whole clue is the definition (& lit or double definition).
How the clue works
This one's a cryptic definition, so the whole of Patsy's date of birth? is the clue, with no separate wordplay to dig out. A patsy is a gullible sort, and the joke leans on the old line "I wasn't born yesterday", the thing you say to insist you're nobody's fool. So the date a patsy might be said to have been born is the one the naive are always accused of.