Conrad is terribly caustic (8)
I believe the answer is:
sardonic
'caustic' is the definition.
(thesaurus)
'conrad is terribly' is the wordplay.
'terribly' is an anagram indicator.
'conrad'+'is'='conradis'
'conradis' with letters rearranged gives 'SARDONIC'.
(Other definitions for sardonic that I've seen before include "Cynical -- so rancid (anag)" , "Mocking or scornful" , "Dry, cynically mocking" , "Disdainfully or ironically humorous" , "Grimly cynical" .)