A beginner in athletics? (7)
I believe the answer is:
starter
'a beginner' is the definition.
(starter is a kind of beginner)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for starter that I've seen before include "No substitute" , "cooking served first?" , "Beginner, or opening course of meal" , "Appetiser" , "One giving signal to begin race" .)