A catastrophe of old (5)
I believe the answer is:
flood
'a catastrophe' is the definition.
The answer and definition can be both types of natural phenomena as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'of old' is the wordplay.
'of old' anagrams to 'FLOOD'.
Though, I'm not sure how this is indicated.
Can you help me to learn more?
(Other definitions for flood that I've seen before include "Inundation" , "lot of water" , "Light" , "Overwhelming amount" , "swamp" .)