Sailor solely after shellfish (7)
I believe the answer is:
abalone
'shellfish' is the definition.
(abalone is a kind of shellfish)
'sailor solely after' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'solely' becomes 'alone' ('alone' can be a synonym of 'solely').
'after' says to put letters next to each other.
'ab'+'alone'='ABALONE'
(Other definitions for abalone that I've seen before include "Edible mollusc - on a bale (anag)" , "Mollusc also called ormer" , "Seafood" , "Edible shellfish, sea-ear" , "Mother-of-pearl could have come from" .)