Mean old girl in court (9)
I believe the answer is:
excellent
'mean' is the definition.
(thesaurus)
'old girl in court' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'girl' becomes 'ellen' ().
'in' means one lot of letters goes inside another.
'court' becomes 'ct' (abbreviation used in road names).
'ellen' put inside 'ct' is 'cellent'.
'ex'+'cellent'='EXCELLENT'
(Other definitions for excellent that I've seen before include "Superior, of the highest quality" , "capital" , "Great!" , "First-class" , "Really good, top class" .)