One bringing luck in racecourse, miles ahead (6)
I believe the answer is:
mascot
'one bringing luck' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'racecourse miles ahead' is the wordplay.
'racecourse' becomes 'ascot' (British racecourse).
'miles' becomes 'm' (common abbreviation).
'ahead' is a charade indicator (letters next to each other) (some letters go in front of others).
'ascot' after 'm' is 'MASCOT'.
'in' is the link.
Can you help me to learn more?
(Other definitions for mascot that I've seen before include "Team's lucky person/thing" , "good luck charm required" , "Lucky charm" , "Lucky thing" , "It is believed to bring good luck to its owner(s)" .)