He's on board for the sport (6)
I believe the answer is:
surfer
This clue might be a double definition.
'he's on board' is the first definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'sport' is the second definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'for the' acts as a link.
Can you help me to learn more?
(Other definitions for surfer that I've seen before include "person online?" , "following 28 [SILVER]" , "Boarder" , "Wave rider" , "See 7" .)