Crossword Genius

The one that is the expert (3)

Ross

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" .)

I've seen this clue in The Globe and Mail.
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