Chain of events with disastrous end: come off it (6,6)
I believe the answer is:
domino effect
'chain of events' is the definition.
(I've seen this before)
'disastrous end come off it' is the wordplay.
'disastrous' indicates an anagram.
'end'+'come'+'off'+'it'='endcomeoffit'
'endcomeoffit' is an anagram of 'DOMINO EFFECT'.
'with' is the link.
(Other definitions for domino effect that I've seen before include "Consequence of one event setting off a chain of others" , "Each event being a consequence of the previous one" , "Chain of events" , "series of knock-ons" , "Consequence of one event setting off a chain reaction" .)