Star hides much being a deranged killer (3-7-3)
I believe the answer is:
sub-machine-gun
'killer' is the definition.
(a submachine gun kills people)
'star hides much being a deranged' is the wordplay.
'star' becomes 'sun' (the Sun is a star).
'hides' means one lot of letters goes inside another.
'deranged' is an anagram indicator.
'much'+'being'+'a'='muchbeinga'
'muchbeinga' anagrammed gives 'ubmachineg'.
'sun' placed around 'ubmachineg' is 'SUB-MACHINE-GUN'.
(Other definitions for sub-machine-gun that I've seen before include "Shooter (from the hip?)" , "Automatic weapon" , "Lightweight automatic firearm" .)