Neither seen nor done much (4)
I believe the answer is:
rare
'much' is the definition.
Both the definition and answer are adjectives. Perhaps you can see an association between them that I don't see?
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for rare that I've seen before include "'Unusual, uncommon (4)'" , "Rear up, being scarce" , "Seldom seen or met" , "not often spotted" , "Might steak be" .)