Start too early (4,3,3)
I believe the answer is:
jump the gun
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">start too early</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for jump the gun that I've seen before include "Come too early out of the starting blocks?" , "Leave too early" , "Act prematurely" , "Be responsible for a false start" , "Start too early" .)