Upside? (3)
'upside?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for top that I've seen before include "Peak; child's toy" , "Peak; beat" , "Finest" , "Children's toy" , "Upper side" .)
(Other definitions for pro that I've seen before include "With qualifications" , "Athelete who plays for pay" , "One doesn't play just for the love of it, in short" , "Golfer" , "Professional; advantage" .)