Crazy mother backing murderer (6)
I believe the answer is:
maniac
'crazy' is the definition.
(I've seen this before)
'mother backing murderer' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'backing' says the letters should be written in reverse.
'murderer' becomes 'cain' (Cain who killed Abel).
'cain' reversed gives 'niac'.
'ma'+'niac'='MANIAC'
(Other definitions for maniac that I've seen before include "Lunatic, madman" , "Crazy person behaving wildly" , "Very reckless person" , "Psycho" , "Can aim (anag.)" .)