Mum ace in rearranged North Run (8)
I believe the answer is:
marathon
'run' is the definition.
(I know that marathon is a type of run)
'mum ace in rearranged north' is the wordplay.
'mum' becomes 'ma' (ma can mean mother).
'ace' becomes 'a'.
'in' indicates putting letters inside.
'rearranged' indicates an anagram.
'north' is an anagram of 'rthon'.
'a' placed within 'rthon' is 'rathon'.
'ma'+'rathon'='MARATHON'
(Other definitions for marathon that I've seen before include "Race of about 26 miles" , "Footrace of approx 26 miles" , "strenuous event" , "Long-distance foot race" , "Olympic event" .)