Waving hello, maniac creates despair (11)
I believe the answer is:
melancholia
'despair' is the definition.
(thesaurus)
'waving hello maniac' is the wordplay.
'waving' indicates an anagram (letters wave around).
'hello'+'maniac'='hellomaniac'
'hellomaniac' anagrammed gives 'MELANCHOLIA'.
'creates' is the link.
(Other definitions for melancholia that I've seen before include "Black dog" , "Pensive sadness" , "Deep long-lasting sadness" , "Feeling of deep sadness" , "Depressed state" .)