Common viper (5)
I believe the answer is:
adder
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">common viper</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for adder that I've seen before include "Totting snake" , "He's making an extension" , "British snake; summer" , "Poisonous Brit. snake" , "One creature" .)