Column about Norway's unused substitute (5)
I believe the answer is:
locum
'substitute' is the definition.
(locum can mean a substitute)
'column about norway's unused' is the wordplay.
'column' becomes 'col'.
'about' is a reversal indicator.
'norway's unused' becomes 'um' (I can't justify this - if you can you should believe this answer much more).
'col' in reverse letter order is 'loc'.
'loc'+'um'='LOCUM'
Can you help me to learn more?
(Other definitions for locum that I've seen before include "Professional temp" , "Stand-in medic" , "A substitute or stand-in doctor" , "one won't be in position for ever!" , "Physician's stand-in" .)