Join board (5)
I believe the answer is:
I believe this is a double definition.
'join' is the first definition.
(I've seen this in other clues)
'board' is the second definition.
(I know that board is a more specific form of the action enter)
(Other definitions for enter that I've seen before include "Go in - set down" , "Move into" , "put in for a race" , "Fruit drink" , "Register formally as a participant" .)