All the worse for being vicious (6)
I believe the answer is:
lethal
'being vicious' is the definition.
(thesaurus)
'all the worse' is the wordplay.
'worse' indicates anagramming the letters (I've seen this in other clues).
'all'+'the'='allthe'
'allthe' is an anagram of 'LETHAL'.
'for' acts as a link.
(Other definitions for lethal that I've seen before include "Deadly or mortal" , "bringing down the curtain" , "very nasty indeed" , "--- Weapon, Mel Gibson film" , "Deadly dangerous" .)