One of two or one in three, roughly (6)
I believe the answer is:
either
'one of two' is the definition.
(I've seen this before)
'one in three roughly' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'roughly' indicates anagramming the letters.
'three' with letters rearranged gives 'ether'.
'i' inserted within 'ether' is 'EITHER'.
'or' is the link.
(Other definitions for either that I've seen before include "Choice of two" , "Word introducing first of two 16s" , "offered a choice" , "No matter which (of two)" , "One of two alternatives" .)