Not one is to be found in there (7)
I believe the answer is:
neither
'not' is the definition.
Both the definition and answer are adverbs. Maybe you can see a link between them that I can't see?
'one is to be found in there' is the wordplay.
'in there' can be anagrammed to 'NEITHER'.
But, I'm unsure how the anagram is indicated.
Can you help me to learn more?
(Other definitions for neither that I've seen before include "Negative word" , "None of the two" , "Also not" , "In three (anag.)" , "And not; nor yet" .)