Runner starts marathon in London, eagerly racing (5)
I believe the answer is:
miler
'runner' is the definition.
(miler is a kind of runner)
'starts marathon in london eagerly racing' is the wordplay.
'starts' says to take the initial letters.
The first letters of 'marathon in london eagerly racing' is 'MILER'.
(Other definitions for miler that I've seen before include "Pre-metric middle-distance runner" , "Runner over a specific distance" , "one who competes in the long run" , "Athlete running 1,760 yards" , "horse" .)