One in favour of being paid? (3)
I believe the answer is:
pro
I believe this clue is a triple definition!
'one in favour of' is the first definition.
(being pro a particular cause)
'being' is the second definition.
(pro is a kind of being)
'paid?' is the third definition.
(I've seen this in other clues)
(Other definitions for pro that I've seen before include "For; non-amateur" , "In short, not an amateur" , "In brief, he's no amateur" , "Favouring the professional" , "Competent person" .)