Summing (6)
I believe the answer is:
adding
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">summing</span>' is the definition.<br/ ><span class="explanation-format__explanation">('add' can be a synonym of 'sum')</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for adding that I've seen before include "Calculating the sum of" , "Totalling" , "increasing it by" , "Appending" , "Contributing" .)