Common vipers (6)
I believe the answer is:
adders
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">common vipers</span>' is the definition.<br/ ><span class="explanation-format__explanation">('adder' can be a synonym of 'common viper')</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for adders that I've seen before include "Do they deal with tots" , "Arithmetic snakes?" , "Small vipers" , "Mathematical reptiles" , "at the zoo?" .)