Declare large city locked in? Not after that! (2,6)

I believe the answer is:

at latest

'not after that' is the definition.
(at the latest means not after that point)

'declare large city locked in?' is the wordplay.
'declare' becomes 'attest' (attesting is a kind of declaring).
'large city' becomes 'LA' (Los Angeles).
'locked in?' means one lot of letters goes inside another.
'attest' going around 'la' is 'AT LATEST'.

I've seen this clue in The Guardian.

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