In favour of backing a poor finisher (3)
I believe the answer is:
for
'in favour' is the definition.
(as in being for a particular cause)
'of backing a poor finisher' is the wordplay.
'backing' shows that the letters should be reversed in order.
'finisher' says to take the final letters (I've seen 'finishers' mean this).
The final letter of 'poor' is 'r'.
'of' backwards is 'fo'.
'fo'+'r'='FOR'
(Other definitions for for that I've seen before include "On behalf of" , "See 24" , "Pro" , "In support of" , "On the side of" .)