Runner beginning to run, one runs! (5)
I believe the answer is:
racer
'runner' is the definition.
(thesaurus)
'beginning to run one runs' is the wordplay.
'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'.
'r'+'ace'+'r'='RACER'
(Other definitions for racer that I've seen before include "Fast driver" , "sportsman" , "Fast competitive car or animal" , "Athlete" , "One car" .)