The one that is the expert (3)
I believe the answer is:
ace
I believe this clue is a double definition.
'the one' is the first definition.
(ace has a value of one in some card games)
'the expert' is the second definition.
(ace can informally mean very good)
'that is' acts as a link.
(Other definitions for ace that I've seen before include "Tennis serve that the receiver can't reach" , "reason for point of no return" , "'wicked'!" , "Hole in one; card" , "One or 11 in cards" .)