Of two things, not one is in there, surprisingly (7)
I believe the answer is:
neither
'of two things not one' is the definition.
I can't tell whether this definition defines the answer.
'in there surprisingly' is the wordplay.
'surprisingly' indicates an anagram (a surprising ordering of the letters).
'in'+'there'='inthere'
'inthere' with letters rearranged gives 'NEITHER'.
'is' is the link.
Can you help me to learn more?
(Other definitions for neither that I've seen before include "Not one of two" , "both are rejected" , "Not one thing or the other" , "Also not" , "Negative word" .)