Not one in three, sadly (7)
I believe the answer is:
neither
'not one in three sadly' is the definition.
I can't tell whether this defines the answer.
'in three sadly' is the wordplay.
'sadly' is an anagram indicator.
'in'+'three'='inthree'
'inthree' is an anagram of 'NEITHER'.
Can you help me to learn more?
(Other definitions for neither that I've seen before include "double negative?" , "None of the two" , "both are rejected" , "Not one thing or the other" , "Not one of two" .)