Seaman, on his tod, finds shellfish (7)
I believe the answer is:
abalone
'shellfish' is the definition.
(abalone is a kind of shellfish)
'seaman on his tod' is the wordplay.
'seaman' becomes 'ab'.
'on his tod' becomes 'alone' (to be on one's tod is to be alone).
'ab'+'alone'='ABALONE'
'finds' is the link.
(Other definitions for abalone that I've seen before include "sea mollusc" , "Edible marine mollusc -- Oban ale (anag)" , "Mollusc, the sea-ear" , "something to eat from the sea" , "Shell used for decoration" .)