One of two that is set up to the right (6)
I believe the answer is:
either
'one of two' is the definition.
(one of two options)
'that is set up to the right' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'set up' is a reversal indicator (in a down clue, letters go up).
'to' says to put letters next to each other.
'right' becomes 'r' (abbreviation).
'ie' back-to-front is 'ei'.
'ei'+'the'+'r'='EITHER'
(Other definitions for either that I've seen before include "One of two alternatives" , "Word introducing first alternative" , "No matter which (of two)" , "Word introducing first of two 16s" , "Her tie (anag.)" .)