British vessel, sailing ship (4)
I believe the answer is:
bark
'sailing ship' is the definition.
(bark is a kind of sailing ship)
'british vessel' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'vessel' becomes 'ark' (ark is a kind of vessel**).
'b'+'ark'='BARK'
(Other definitions for bark that I've seen before include "Cry; surface of tree" , "Cry of a dog or fox" , "Sound made by a dog" , "Sound from animal < having part of tree" , "Tree's outer layer" .)