A maiden, improperly (5)
I believe the answer is:
amiss
'improperly' is the definition.
(I've seen this in another clue)
'a maiden' is the wordplay.
'maiden' becomes 'miss' (I've seen this before).
'a'+'miss'='AMISS'
(Other definitions for amiss that I've seen before include "Unsuitable" , "Wrong (a girl!)" , "Fallacious" , "Wrong - awry" , "inappropriately" .)