Person standing in for some drunken columnist? (5)

I believe the answer is:

locum

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

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

(Other definitions for locum that I've seen before include "Stand-in professional" , "One standing in" , "Stand-in GP" , "Physician's stand-in" , "He fills in" .)

I've seen this clue in the Evening Standard.

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