Runner beginning to run, one runs! (5)
I believe the answer is:
Here is my best explanation:
'runner' is the definition.
(I've seen this before)
'beginning to run one runs' is the subsidiary indication.
'beginning to' indicates taking the first letters.
'one' becomes 'ace' ('ace' can be a synonym of 'one').
'runs' becomes 'r' (cricket abbreviation).
The initial letter of 'run' is 'r'.
(Other definitions for racer that I've seen before include "Athlete", "One car", "sportsman", "Fast competitive car or animal", "Run competitor; snake", "fast car", "Participant in speed contest", "Bike, perhaps", "Fast-moving snake" and "Fast mover".)