A substitute for medicine (5)
I believe the answer is:
'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" .)