Quarry Etna? Ludicrous, according to geologists nowadays (10)
I believe the answer is:
quaternary
'geologists nowadays' is the definition.
The definition and answer can be both related to time as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'quarry etna? ludicrous' is the wordplay.
'ludicrous' indicates an anagram (a ludicrous spelling).
'quarry'+'etna'='quarryetna'
'quarryetna' with letters rearranged gives 'QUATERNARY'.
'according to' acts as a link.
Can you help me to learn more?
(Another definition for quaternary that I've seen is " Set of 4".)