At sea, Walter Raleigh's first vessel (7)
I believe the answer is:
trawler
'vessel' is the definition.
(I know that Trawler is a type of fishing ship)
'at sea walter raleigh's first' is the wordplay.
'at sea' indicates an anagram.
'raleigh's first' becomes 'r' (1st letter of 'raleigh').
'walter' anagrammed gives 'trawle'.
'trawle'+'r'='TRAWLER'
(Other definitions for trawler that I've seen before include "Fishing-boat" , "Fishing vessel that drags nets behind it" , "seagoing vessel" , "Boat using a net to catch fish" , "One searching" .)