Crosswise on deck (5)
I believe the answer is:
abeam
'crosswise on deck' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for abeam that I've seen before include "across width of ship" , "neither fore nor aft?" , "To the right or left of the centre of the ship" , "across the deck" , "At right-angles to keel" .)