Virulent (5)
I believe the answer is:
toxic
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">virulent</span>' is the definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for toxic that I've seen before include "Deadly - venomous" , "word describing bad things" , "Poisonous, noxious" , "Harmful - cox it (anag)" .)