Not even one of two in three is out of order (7)
I believe the answer is:
neither
'not even one of two' is the definition.
(neither of two options)
'in three is out of order' is the wordplay.
'is out of order' indicates anagramming the letters.
'in'+'three'='inthree'
'inthree' anagrammed gives 'NEITHER'.
(Other definitions for neither that I've seen before include "Negative word" , "Also not" , "double negative?" , "In three (anag.)" , "both are rejected" .)