Is she the wrong side of forty? (5)

I believe the answer is:

hetty

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'is she' is the definition.

'the wrong side of forty?' is the wordplay.
'wrong' indicates anagramming the letters.
'side of forty?' becomes 'ty' (I am not sure about this - if you are sure you should believe this answer much more).
'the' is an anagram of 'het'.
'het'+'ty'='HETTY'

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I've seen this clue in the Evening Standard.

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