Rich pet at sea in vessel (7)
I believe the answer is:
pitcher
'vessel' is the definition.
(pitcher is a kind of vessel)
'rich pet at sea' is the wordplay.
'at sea' indicates anagramming the letters.
'rich'+'pet'='richpet'
'richpet' is an anagram of 'PITCHER'.
'in' acts as a link.
(Other definitions for pitcher that I've seen before include "Baseball bowler and container for liquid" , "Does one throw the vessel?" , "Baseball thrower or water jug" , "American sportsman" , "one tries to sell in a market?" .)