Seaman, unaccompanied, finds shellfish (7)
I believe the answer is:
abalone
'shellfish' is the definition.
(abalone is a kind of shellfish)
'seaman unaccompanied' is the wordplay.
'seaman' becomes 'ab'.
'unaccompanied' becomes 'alone' ('alone' can be a synonym of 'unaccompanied').
'ab'+'alone'='ABALONE'
'finds' is the link.
(Other definitions for abalone that I've seen before include "One lives in a shell" , "Would you be able on a US shellfish?" , "something to eat from the sea" , "Shell used for decoration" , "Mollusc, the sea-ear" .)