Use a stopwatch (4)
I believe the answer is:
time
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">use a stopwatch</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 time that I've seen before include "Hours clocked up?" , "Prison sentence; tempo" , "Pub closing call" , "> moment" , "It's done in prison" .)