Psycho killer rejected by mother (6)
I believe the answer is:
maniac
'psycho' is the definition.
(I've seen this before)
'killer rejected by mother' is the wordplay.
'killer' becomes 'cain' (Cain who killed Abel).
'rejected' says the letters should be written in reverse.
'by' says to put letters next to each other.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'cain' written backwards gives 'niac'.
'niac' after 'ma' is 'MANIAC'.
(Other definitions for maniac that I've seen before include "Very reckless person" , "Can aim (anag.)" , "Mad (keen) type" , "fruitcake?" , "Crazy person behaving wildly" .)