Throw in (3)
I believe the answer is:
add
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">throw in</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 add that I've seen before include "Contribute" , "Include" , "Count up" , "Say as a further remark" , "Give more" .)