Adder (5)
I believe the answer is:
viper
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">adder</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for viper that I've seen before include "poisonous type" , "Malicious person" , "Adder" , "Malicious type" , "A poisonous snake or treacherous person" .)