He's about six, you know! (5)

I believe the answer is:

kevin

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'he's about six you know' is the definition.
The definition suggests a singular noun which matches the answer.

'he's about six' is the wordplay.
'he' becomes 'ken' ().
'about' is an insertion indicator.
'six' becomes 'VI' (Roman numerals).
'ken' enclosing 'vi' is 'KEVIN'.

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(Other definitions for kevin that I've seen before include "Mr Keegan?" , "- Costner, US actor (Dances with Wolves)" , "Man's name" .)

I've seen this clue in the Evening Standard.

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