A substitute for some indisposed columnist? (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 rest of the clue works.

(Other definitions for locum that I've seen before include "Someone standing in temporarily for another of the same profession" , "Medical temp?" , "Professional who stands in for another" , "Stand-in, for a doctor say" , "Deputising doctor" .)

I've seen this clue in the Evening Standard.

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