Tot up (3)
I believe the answer is:
add
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">tot up</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for add that I've seen before include "Augment" , "Reason for difficulty learning" , "Give more" , "Put together - append" , "Sum up; say further" .)