Stumble upon hit (3,4)
I believe the answer is:
run into
I believe this clue is a double definition.
'stumble upon' is the first definition.
(thesaurus)
'hit' is the second definition.
(running into is a kind of hitting)
(Other definitions for run into that I've seen before include "Encounter (someone)" , "Encounter by chance - or collision?" , "Meet (someone) unexpectedly" , "Happen to meet or collide with" , "Meet accidentally on the trot?" .)