Substitute, initially left out, controversially undertaking marathon (5)
I believe the answer is:
locum
'substitute' is the definition.
(locum can mean a substitute)
'initially left out controversially undertaking marathon' is the wordplay.
'initially' indicates taking the first letters.
The first letters of 'left out controversially undertaking marathon' is 'LOCUM'.
(Other definitions for locum that I've seen before include "One standing in" , "Professional who stands in for another" , "Physician's stand-in" , "Temporary replacement doctor" , "Professional who substitutes temporarily for another" .)