Substitute ballplayers for pacey but outnumbered centres (5)

I believe the answer is:

locum

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'substitute' is the definition.
(locum is a kind of substitute)

I cannot understand how the rest of the clue works.

(Other definitions for locum that I've seen before include "Physician's stand-in" , "Temporary replacement doctor" , "Doctor's temporary replacement" , "One stands in" , "He fills in" .)

I've seen this clue in The Independent.

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