United sailors in vessel (3)
I believe the answer is:
urn
'vessel' is the definition.
(urn is a kind of vessel)
'united sailors' is the wordplay.
'united' becomes 'u'.
'sailors' becomes 'RN' (abbreviation for Royal Navy).
'u'+'rn'='URN'
'in' acts as a link.
(Other definitions for urn that I've seen before include "Source of tea" , "Samovar, perhaps" , "Pot, vase" , "Tea vessel or ash container" , "Vaselike vessel on a pedestal" .)