European vipers (6)
I believe the answer is:
adders
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">european vipers</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for adders that I've seen before include "Totting snakes" , "Arithmetic snakes?" , "Re Dad's snakes" , "Snakes are totters" , "Small vipers" .)