Say further (3)
I believe the answer is:
add
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">say further</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 add that I've seen before include "Sum, put together" , "do some mathematics" , "problem for hyperactive children" , "Aggregate or supplement" , "Sum up; say further" .)