Suddenly lose patience (4)
I believe the answer is:
snap
'suddenly lose patience' 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 "Break sharply like dry twig" , "Be irritable at the card game" , "that's a copy!" , "Break - children's card game" , "Speak irritably; break" .)