Chime in (3)
I believe the answer is:
add
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">chime in</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for add that I've seen before include "Reckon" , "Determine the sum of" , "Go further" , "what a computer can do" , "do some mathematics" .)