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' says the letters should be written in reverse (in a down clue, the letters are written upwards).
'to' says to put letters next to each other.
'end' indicates one should take the final letters.
The final letter of 'far' is 'r'.
'of' in reverse letter order is 'fo'.
'fo'+'r'='FOR'
(Other definitions for for that I've seen before include "On behalf of" , "In support of" , "See 24" , "Approving" , "Favouring" .)