Not one therein, perhaps (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 therein perhaps' is the wordplay.
'perhaps' indicates anagramming the letters.
'therein' anagrammed gives 'NEITHER'.
Can you help me to learn more?
(Other definitions for neither that I've seen before include "Also not" , "The rein (anag.)" , "Not one of two" , "Not one thing or the other" , "Negative word" .)