Official on course or attendant in mess? (7)
I believe the answer is:
steward
This clue is a double definition.
'official on course' is the first definition.
(sporting steward)
'attendant in mess?' is the second definition.
(as in a ship's mess)
'or' acts as a link.
(Other definitions for steward that I've seen before include "Attendant on plane or ship" , "Caretaker" , "Superintendent" , "Ship/aircraft attendant" , "Horse-racing official" .)