Bosom hoisted by parting of the ways (4-2)
I believe the answer is:
bust-up
'parting of the ways' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'bosom hoisted' is the wordplay.
'bosom' becomes 'bust' (I've seen this before).
'hoisted' becomes 'up' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'bust'+'up'='BUST-UP'
'by' is the link.
Can you help me to learn more?
(Other definitions for bust-up that I've seen before include "Serious altercation" , "Serious quarrel or fight (B, not D)" , "Argument finishing a relationship" , "Serious quarrel/fight" , "disagreement" .)