Baseball player’s casual two-piece outfit? (9)

I believe the answer is:

shortstop

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'baseball player's' is the definition.
(shortstop is a kind of baseball player)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for shortstop that I've seen before include "Fielder in baseball" , "Fielding position (baseball)" , "US player" , "One in field" , "position in American sport" .)

I've seen this clue in The Times.

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