Act prematurely (4,3,3)
I believe the answer is:
jump the gun
'act prematurely' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for jump the gun that I've seen before include "Come too early out of the starting blocks?" , "Start prematurely" , "Leave too early" , "Act prematurely" , "Start too early" .)