Psychotic first killer America briefly rejected (6)
I believe the answer is:
maniac
'psychotic' is the definition.
(thesaurus)
'first killer america briefly rejected' is the wordplay.
'first' says to put letters next to each other.
'killer' becomes 'cain' (Cain who killed Abel).
'america briefly' becomes 'am' ('briefly' asks for a short form).
'rejected' says the letters should be written in reverse.
'cain' put next to 'am' is 'cainam'.
'cainam' backwards is 'MANIAC'.
(Other definitions for maniac that I've seen before include "Lunatic or person behaving wildly" , "Crazy person behaving wildly" , "Psychopath" , "Mad (keen) type" , "Psycho" .)