British snake (5)
I believe the answer is:
adder
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">british snake</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for adder that I've seen before include "Venomous type" , "Britain's only venomous snake" , "Small poisonous snake, viper" , "venomous creature" , "An arithmetic snake" .)