A group of directors on a boat (6)
I believe the answer is:
aboard
'on a boat' is the definition.
(aboard a boat)
'a group of directors' is the wordplay.
'group of directors' becomes 'board' (board of directors).
'a'+'board'='ABOARD'
(Other definitions for aboard that I've seen before include "on boat, perhaps" , "On (a ship, train, etc)" , "On (a ship/plane)" , "Part of the group" , "'On a ship or train, say (6)'" .)