Cruel tyrant kills editor on return (6)
I believe the answer is:
despot
'cruel tyrant' is the definition.
(a despot is a tyrant or dictator)
'kills editor on return' is the wordplay.
'kills' becomes 'tops'.
'editor' becomes 'ed' (abbreviation).
'on return' shows that the letters should be reversed in order.
'tops'+'ed'='topsed'
'topsed' written backwards gives 'DESPOT'.
(Other definitions for despot that I've seen before include "nasty ruler?" , "Gaddafi, say" , "Cruel and oppressive dictator" , "Louis XVI was one" , "One exploiting rule" .)