Vessel, a sort of boat for the navy (3)
I believe the answer is:
urn
'vessel' is the definition.
(urn is a kind of vessel)
'boat for the navy' is the wordplay.
'boat' becomes 'u' ().
'for' says to put letters next to each other.
'the navy' becomes 'rn' (abbreviation for Royal Navy).
'u'+'rn'='URN'
'a sort of' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for urn that I've seen before include "Pot for human ashes" , "Large ornamental vase" , "Tea vessel or ash container" , "Water vessel" , "a large vase as for cremation ashes" .)