Temporary doctor (5)
I believe the answer is:
locum
'temporary doctor' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for locum that I've seen before include "Professional who substitutes temporarily for another" , "Someone standing in temporarily for another of the same profession" , "may step into someone else's job" , "Temporary deputy for doctor" , "One standing in" .)