Tie up horse east of rivers, both one and the other (6)
I believe the answer is:
either
'both one and the other' is the definition.
I can't judge whether this defines the answer.
'tie up horse east of rivers' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'horse' becomes 'H' (abbreviation as in horse power).
'east' becomes 'e' (abbreviation).
'of rivers' becomes 'r' (I can't explain this - if you can you should believe this answer much more).
'tie' in reverse letter order is 'eit'.
'eit'+'h'+'e'+'r'='EITHER'
Can you help me to learn more?
(Other definitions for either that I've seen before include "could be one of two" , "No matter which (of two)" , "Her tie (anag.)" , "One of two alternatives" , "Choice of two" .)