Lucky charm Mark found on racecourse (6)
I believe the answer is:
mascot
'lucky charm' is the definition.
(something bringing good luck)
'mark found on racecourse' is the wordplay.
'mark' becomes 'm'.
'found on' says to put letters next to each other.
'racecourse' becomes 'ascot' (British racecourse).
'm'+'ascot'='MASCOT'
(Other definitions for mascot that I've seen before include "Team's good-luck bringer" , "Does it get you first place" , "Luck-bringer" , "It's adopted by a team as a symbolic figure" , "Team's lucky person/thing" .)