Expert on board relative's trailer when train missed (11)
I believe the answer is:
grandmaster
'expert on board' is the definition.
(expert at chess - a board game)
I can't explain the rest of the clue.
(Other definitions for grandmaster that I've seen before include "Star chess player" , "a wizard with his pieces?" , "A top chess player" , "Expert on board" , "Chess player of the highest class" .)