Bound, at that point, to arrest half of guys, but act too soon (4,3,3)
I believe the answer is:
jump the gun
'act too soon' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'bound at that point to arrest half of guys' is the wordplay.
'bound' becomes 'jump' (I've seen this in another clue).
'at that point' becomes 'then' (I've seen this in another clue).
'to arrest' is an insertion indicator (inserted letters are arrested or held).
'half of guys' becomes 'gu' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'then' going around 'gu' is 'thegun'.
'jump'+'thegun'='JUMP THE GUN'
'but' is the link.
Can you help me to learn more?
(Other definitions for jump the gun that I've seen before include "Start too quickly" , "Be responsible for a false start" , "Leave too early" , "Act prematurely" , "Come too early out of the starting blocks?" .)