Serious quarrel (4-2)
I believe the answer is:
bust-up
'serious quarrel' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for bust-up that I've seen before include "Argument finishing a relationship" , "Quarrel, brawl" , "Quarrel, possibly ending a friendship" , "Serious quarrel or fight (B, not D)" , "Serious altercation" .)