Vessel, a sort of boat for the navy (3)
I believe the answer is:
urn
'vessel' is the definition.
(urn is a type of container)
'boat for the navy' is the wordplay.
'boat' becomes 'u' ().
'for' means one lot of letters go next to another.
'the navy' becomes 'RN' (Royal Navy).
'u'+'rn'='URN'
'a sort of' is the link.
I am not very happy about this link. Some or all of it may be part of 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" .)