A substitute for medicine (5)

I believe the answer is:

locum

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'a substitute' is the definition.
(locum can mean a substitute)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for locum that I've seen before include "Relief doctor" , "Person filling in" , "Stand-in professional" , "Professional who stands in for another" , "He fills in" .)

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

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