Friend on board (4)
I believe the answer is:
mate
This might be a double definition.
'friend' is the first definition.
(I know that mate is a type of friend)
'board' is the second definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'on' acts as a link.
Can you help me to learn more?
(Other definitions for mate that I've seen before include "Better half" , "declared winning position on board" , "Team up with partner" , "One of a pair or a pal" , "Friend, buddy" .)