Fire on all cylinders (3)
I believe the answer is:
hum
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fire on all cylinders</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 hum that I've seen before include "Drone - stink" , "Sound of busy activity" , "Reek" , "Stench" , "Buzz; stink" .)