Maniac chops yuppie's head off (6)
I believe the answer is:
psycho
'maniac' is the definition.
'chops yuppie's head off' is the wordplay.
'head' says to take the initial letters.
'off' is an anagram indicator.
The first letter of 'yuppie' is 'y'.
'chops'+'y'='chopsy'
'chopsy' with letters rearranged gives 'PSYCHO'.
(Other definitions for psycho that I've seen before include "Nineteen sixty Hitchcock film" , "Method of treatment pioneered by Freud" , "Loony" , "Prefix denoting the mind or soul" , "In brief, person with severe mental disorder - ask Alfred Hitchcock" .)