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.
Perhaps they are linked in a way I don't understand?
'of old' is the wordplay.
'of old' anagrams to 'FLOOD'.
However, I am not sure how the anagram 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" .)