A young lady at fault (5)

I believe the answer is:

amiss

'at fault' is the definition.
(thesaurus)

'a young lady' is the wordplay.
'young lady' becomes 'miss' ('miss' can be a synonym of 'young lady'**).
'a'+'miss'='AMISS'

(Other definitions for amiss that I've seen before include "Taking wrong turn" , "Haywire" , "inappropriately" , "astray" , "Fallacious" .)

I've seen this clue in The Telegraph.

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