Knocked off witty number (5-3)

I believe the answer is:

fifty-two

'number' is the definition.
(fifty-two is a number)

'knocked off witty' is the wordplay.
'knocked' indicates anagramming the letters.
'off'+'witty'='offwitty'
'offwitty' anagrammed gives 'FIFTY-TWO'.

(Another definition for fifty-two that I've seen is " Number of playing cards in a pack".)

I've seen this clue in The Times.

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