A number of kittens (3)

I believe the answer is:

ten

'a number' is the definition.
(ten is a number)

'of kittens' is the wordplay.
'of' indicates the answer is hidden within the clue.
'TEN' is hidden in the letters of 'kittens'.

(Other definitions for ten that I've seen before include "How many" , "X-WORD" , "time" , "Base of decimal system" , "PM's address, Downing St" .)

I've seen this clue in the Evening Standard.

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