Underworld girl in sack (7)
I believe the answer is:
dismiss
'sack' is the definition.
(I know that sack can be written as dismiss)
'underworld girl' is the wordplay.
'underworld' becomes 'dis' (underworld in classical mythology).
'girl' becomes 'miss' (term of address for an unmarried woman).
'dis'+'miss'='DISMISS'
'in' acts as a link.
(Other definitions for dismiss that I've seen before include "Cease to consider" , "Reject, send away" , "Drop" , "Sack (from job)" , "Remove from office" .)