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", "Fast mover", "Run competitor; snake", "Fast-moving snake", "Fast competitive car or animal", "Participant in speed contest", "Fast-moving animal" and "Runner".)