It was long ago, but not quite antediluvian (6)
I believe the answer is:
deluge
'ago but not quite antediluvian' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'it was long' is the wordplay.
I cannot quite see how this works, but
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is present in the answer.
'long' could be 'l' and 'l' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for deluge that I've seen before include "A heavy downpour of rain" , "Huge downpour" , "A heavy fall of rain (6)" , "Severe flood" , "Flood - downpour" .)