Not one or the other I found in underground (7)
I believe the answer is:
neither
'not one or the other' is the definition.
(I've seen this in another clue)
'i found in underground' is the wordplay.
'found in' means one lot of letters goes inside another.
'underground' becomes 'nether' (I've seen this before).
'i' going inside 'nether' is 'NEITHER'.
(Other definitions for neither that I've seen before include "Therein lies not one or the other" , "Not one thing or the other" , "double negative?" , "In three (anag.)" , "both are rejected" .)