In favour of going up to the far end (3)
I believe the answer is:
for
'in favour' is the definition.
(as in being for a particular cause)
'of going up to the far end' is the wordplay.
'going up' shows that the letters should be reversed in order (in a down clue, the letters are written upwards).
'to' is a charade indicator (letters next to each other).
'end' says to take the final letters.
The final letter of 'far' is 'r'.
'of' reversed gives 'fo'.
'fo'+'r'='FOR'
(Other definitions for for that I've seen before include "On behalf of" , "In support of" , "See 24" , "Approving" , "Favouring" .)