Start too early (4,3,3)
I believe the answer is:
jump the gun
'start too early' is the definition.
(I've seen this before)
This is the entire clue.
(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" .)