Not one is to be found in there (7)
I believe the answer is:
neither
'not' is the definition.
Both the answer and definition are adverbs. Maybe they are linked in a way I don't understand?
'one is to be found in there' is the wordplay.
'in there' can be anagrammed to 'NEITHER'.
But, I am not clear 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" .)