Improper young lady with ace top (5)
I believe the answer is:
amiss
'improper' is the definition.
(thesaurus)
'young lady with ace top' is the wordplay.
'young lady' becomes 'miss' ('miss' can be a synonym of 'young lady'**).
'with' says to put letters next to each other.
'top' indicates taking the first letters.
The first letter of 'ace' is 'a'.
'miss' after 'a' is 'AMISS'.
(Other definitions for amiss that I've seen before include "(Gone) wrong" , "Wrong; inaccurate" , "Badly - wrong" , "Wrongly" , "Revealing fault" .)