Onset (5)
I believe the answer is:
start
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">onset</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 start that I've seen before include "Onset" , "lead" , "open" , "Initiate" , "Jerk" .)