Go off the deep end (4)
I believe the answer is:
snap
'go off the deep end' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for snap that I've seen before include "Crack; bite" , "Break like simple card game" , "Speak in a sharp angry tone" , "Break suddenly and cleanly" , "Become unhinged" .)