One in three, otherwise one of two (6)
I believe the answer is:
either
'one of two' is the definition.
(I've seen this before)
'one in three otherwise' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'otherwise' indicates anagramming the letters.
'three' with letters rearranged gives 'ether'.
'i' inserted into 'ether' is 'EITHER'.
(Other definitions for either that I've seen before include "Word introducing first alternative" , "Choice of two" , "Word introducing first of two 16s" , "could be one of two" , "Word indicating the following alternatives" .)