Start to cauterise a blood vessel and collapse (4,2)
I believe the answer is:
cave in
'collapse' is the definition.
(synonyms)
'start to cauterise a blood vessel' is the wordplay.
'start to' says to take the initial letters.
'blood vessel' becomes 'vein' (vein is a kind of blood vessel).
The first letter of 'cauterise' is 'c'.
'c'+'a'+'vein'='CAVE-IN'
'and' acts as a link.
(Other definitions for cave in that I've seen before include "(Roof) collapse; abandon resistance" , "Yield to outside pressure" , "(Opposition, roof) collapse" , "Lose willpower" , "Subside, collapse" .)