Nothing on earth could be so described (12)
I believe the answer is:
subterranean
'described' is the definition.
Both the definition and answer are adjectives. Perhaps they are linked in a way I don't understand?
'nothing on earth could be' is the wordplay.
I cannot quite see how this works, but
'earth' could be 'terra' (I've seen this in another clue) and 'terra' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'so' acts as a link.
Can you help me to learn more?
(Other definitions for subterranean that I've seen before include "Underground; secret" , "beneath the waves" .)