British sailors in American vessels (4)
I believe the answer is:
urns
'vessels' is the definition.
(urn is a kind of vessel)
'british sailors in american' is the wordplay.
'british sailors' becomes 'RN' (Royal Navy).
'in' indicates putting letters inside.
'american' becomes 'US' (United States).
'rn' inserted into 'us' is 'URNS'.
(Other definitions for urns that I've seen before include "Containers for ashes" , "Grecian vases" , "Runs to find large vases" , "Tall vases" , "Burial vases" .)