Stand-in doctor (5)
I believe the answer is:
'stand-in doctor' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for locum that I've seen before include "temporary occupant" , "one won't be in position for ever!" , "A substitute or stand-in doctor" , "Stand-in, for a doctor say" , "Doctor's deputy" .)