Not one of two or three in disorder (7)
I believe the answer is:
neither
'not one of two' is the definition.
(I've seen this before)
'three in disorder' is the wordplay.
'disorder' is an anagram indicator (a disordered version of the letters).
'three'+'in'='threein'
'threein' with letters rearranged gives 'NEITHER'.
'or' acts as a link.
(Other definitions for neither that I've seen before include "Negative word" , "Not one thing or the other" , "Therein lies not one or the other" , "And not; nor yet" , "Not one of two" .)