Breaks open suddenly (6)
I believe the answer is:
bursts
'breaks open suddenly' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for bursts that I've seen before include "Explodes or breaks open" , "Explodes or ruptures" , "Problems with water pipes" , "Breaks open suddenly" , "Explodes from inside pressure like balloon" .)