1 of 2 or 1 in 3, roughly? (6)
I believe the answer is:
either
'1 of 2' is the definition.
(either of the two)
I don't understand how the rest of the clue works.
(Other definitions for either that I've seen before include "offered a choice" , "Her tie (anag.)" , "One of two alternatives" , "Each of two; moreover" , "Word introducing first of two 16s" .)