Doctor at no harm in the long run (8)
I believe the answer is:
marathon
'the long run' is the definition.
(long-distance run)
'doctor at no harm' is the wordplay.
'doctor' is an anagram indicator.
'at'+'no'+'harm'='atnoharm'
'atnoharm' with letters rearranged gives 'MARATHON'.
'in' acts as a link.
(Other definitions for marathon that I've seen before include "no easy task" , "Race; battle" , "The longest foot race in sport" , "26-mile foot race" , "Long distance race" .)