In favor of (3)
I think it could be either:
'in favor of' 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 "Top player" , "Competent person" , "Expert (abbr)" , "Athelete who plays for pay" , "Preposition meaning for, in favour of" .)
'in favor of' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for for that I've seen before include "On the side of" , "and 16 and 20: Notwithstanding" , "In favour of; since" , "Favouring" , "On behalf of" .)