Two's a little too late to be in top form (2,4,4)

I believe the answer is:

at ones best

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'in top form' is the definition.
(I've seen this in another clue)

'two's a little too late to be' is the wordplay.
I cannot quite see how this works, but
'a' could be 'one' and 'one' is found within the answer.
'be' is present in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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