Doctor taking holiday: that's a relief (5)
I believe the answer is:
locum
This clue could be a double definition.
'doctor taking holiday' is the first definition.
'locum' can be an answer for 'doctor' (I've seen this before). I'm not sure about the 'taking holiday' bit.
'a relief' is the second definition.
(locum is a kind of relief)
'that's' is the link.
Can you help me to learn more?
(Other definitions for locum that I've seen before include "one won't be in position for ever!" , "Temporary deputy for doctor" , "Professional temp" , "Medical temp?" , "Physician's stand-in" .)