Reliable pro (3)
I believe the answer is:
ace
'reliable pro' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ace that I've seen before include "rare driving result?" , "Single unit on playing cards" , "Winning serve; hole in one" , "Tennis serve that the receiver can't reach" , "Card with a single pip" .)