Greek expert on the Underworld put to shame (8)
I believe the answer is:
disgrace
'put to shame' is the definition.
(thesaurus)
'greek expert on the underworld' is the wordplay.
'greek' becomes 'gr' (abbreviation for Greek).
'expert' becomes 'ace' (ace can informally mean very good).
'on' means one lot of letters go next to another.
'the underworld' becomes 'dis' (underworld in classical mythology).
'gr'+'ace'='grace'
'grace' put after 'dis' is 'DISGRACE'.
(Other definitions for disgrace that I've seen before include "State of dishonour" , "Cause of shame or dishonour" , "Loss of reputation as the result of dishonourable action" , "Loss of honour" , "Shame or loss of respect" .)