Expert's unbeatable service (3)

I believe the answer is:

ace

I believe this is a double definition.

'expert's' is the first definition.
(ace can informally mean very good)

'unbeatable service' is the second definition.
(I've seen this in another clue)

(Other definitions for ace that I've seen before include "Tennis serve that the opponent can't reach" , "rare driving result?" , "One in a suit" , "single > club > champion" , "golfer's achievement" .)

I've seen this clue in The Times.

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