Girl shown up as seductress (7)
I believe the answer is:
nerissa
'girl' is the definition.
'shown up as seductress' is the wordplay.
'shown up' is a reversal indicator.
'seductress' becomes 'siren' (I've seen this before).
'as'+'siren'='assiren'
'assiren' backwards is 'NERISSA'.
(Other definitions for nerissa that I've seen before include "One working for Portia" , "Lawyer's clerk apparently" , "Portia's attendant" , "Serving maid" , "Girl from Belmont" .)