Rooms on board (6)
I believe the answer is:
cabins
'rooms on board' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for cabins that I've seen before include "Accommodation on board" , "Wooden huts" , "Small wooden houses like Uncle Tom's" , "Small dwellings often made of wood" , "Rooms on board ship or plane" .)