Favorable factor (3)
I believe the answer is:
pro
'favorable factor' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pro that I've seen before include "Golf expert" , "gets paid to play?" , "Of argument, say, in favour of" , "In brief, he's no amateur" , "Preposition meaning for, in favour of" .)