Not one can be found therein! (7)
I believe the answer is:
neither
'not' is the definition.
Both the definition and answer are adverbs. Perhaps you can see a link between them that I don't see?
'one can be found therein' is the wordplay.
'therein' anagrams to 'NEITHER'.
Though, I am not clear how this is indicated.
Can you help me to learn more?
(Other definitions for neither that I've seen before include "The rein (anag.)" , "double negative?" , "Therein (anag)" , "In three (anag.)" , "Negative word" .)