Runs well (4)
I believe the answer is:
hums
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">runs well</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 hums that I've seen before include "Sings wordlessly" , "Sings with closed lips" .)