Demolish in a fight (4-2)
I believe the answer is:
bust-up
This clue is a double definition.
'demolish' is the first definition.
(I've seen this in another clue)
'a fight' is the second definition.
(I've seen this before)
'in' acts as a link.
(Other definitions for bust-up that I've seen before include "disagreement" , "Quarrel, possibly ending a friendship" , "Serious quarrel or fight (B, not D)" , "Serious quarrel/fight" , "Quarrel, brawl" .)