In favour of not being an amateur (3)

I believe the answer is:

pro

I'm a little stuck... Click here to teach me more about this clue!

'in favour of' is the definition.
(being pro a particular cause)

I cannot understand how the rest of the clue works.

(Other definitions for pro that I've seen before include "With qualifications" , "Golfer" , "Top player" , "In brief, he's no amateur" , "In favour of; advantage" .)

I've seen this clue in The Irish Independent.

Want a hint initially instead of a full solution? Install my app