Not one found therein, perhaps? (7)
I believe the answer is:
neither
'not one found' is the definition.
I can't tell whether this defines the answer.
'therein perhaps?' is the wordplay.
'perhaps?' indicates an anagram (I've seen 'perhaps' mean this).
'therein' with letters rearranged gives 'NEITHER'.
Can you help me to learn more?
(Other definitions for neither that I've seen before include "And not; nor yet" , "Therein lies not one or the other" , "None of the two" , "Therein (anag)" , "Negative word" .)