Underworld character in ditch (7)
I believe the answer is:
discard
'ditch' is the definition.
(I know that ditch is a more specific form of abandon)
'underworld character' is the wordplay.
'underworld' becomes 'dis' (underworld in classical mythology).
'character' becomes 'card' (both can mean a humorous person).
'dis'+'card'='DISCARD'
'in' is the link.
(Other definitions for discard that I've seen before include "Get rid of" , "bin" , "Reject" , "Throw away or cast off" , "Shed" .)