First of murderers - Cain - wild and frenzied (5)
I believe the answer is:
manic
'frenzied' is the definition.
(I know that frenzied can be written as manic)
'first of murderers cain wild' is the wordplay.
'first of murderers' becomes 'm' (1st letter of 'murderers').
'wild' indicates an anagram.
'cain' is an anagram of 'anic'.
'm'+'anic'='MANIC'
'and' acts as a link.
(Other definitions for manic that I've seen before include "hyperactive?" , "demented" , "Deranged" , "Hectic" , "Wildly energetic" .)