One in three, possibly, or one of two (6)
I believe the answer is:
either
'one of two' is the definition.
(I've seen this in another clue)
'one in three possibly' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'possibly' is an anagram indicator.
'three' is an anagram of 'ether'.
'i' placed into 'ether' is 'EITHER'.
'or' is the link.
(Other definitions for either that I've seen before include "Word introducing first of two 16s" , "Word indicating the following alternatives" , "Her tie (anag.)" , "Choice of two" , "offered a choice" .)