Million on course? That's a lucky thing (6)
I believe the answer is:
mascot
'lucky thing' is the definition.
(I've seen this before)
'million on course?' is the wordplay.
'million' becomes 'm' (abbreviation).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'course?' becomes 'Ascot' (Ascot is a racecourse).
'm'+'ascot'='MASCOT'
'that's a' acts as a link.
(Other definitions for mascot that I've seen before include "Symbolic lucky figure" , "Thing believed to bring good luck to, say, team" , "that's charming?" , "Something considered to bring good luck" , "Does it get you first place" .)