Said unfortunately the rest had collapsed: a catastrophe (8)
I believe the answer is:
disaster
'a catastrophe' is the definition.
('disaster' can be a synonym of 'catastrophe')
'said unfortunately the rest had collapsed' is the wordplay.
I cannot really understand how this works, but
an anagram of 'said' is 'disa' which is within the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
The remaining letters 'ser' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for disaster that I've seen before include "Catastrophe, tragedy" , "Sudden catastrophe" , "Heavy blow" , "Catastrophe, calamity" , "Calamity, tragedy" .)