In favour of person getting paid (3)
I believe the answer is:
pro
I believe this is a double definition.
'in favour of' is the first definition.
(being pro a particular cause)
'person getting paid' is the second definition.
(paid professional)
(Other definitions for pro that I've seen before include "Paid worker" , "With qualifications" , "Preposition meaning for, in favour of" , "Of argument, say, in favour of" , "Favouring the professional" .)