Races, as an engine (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">races as an engine</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 revs that I've seen before include "Rotations of a motor, in short" , "Units of engine speed (abbr)" .)