Sailor gets a single shellfish (7)
I believe the answer is:
abalone
'shellfish' is the definition.
(abalone is a kind of shellfish)
'sailor gets a single' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'gets' is a charade indicator (letters next to each other).
'a single' becomes 'alone' (I've seen this before).
'ab'+'alone'='ABALONE'
(Other definitions for abalone that I've seen before include "Edible shellfish, sea-ear" , "Would you be able on a US shellfish?" , "Edible marine mollusc -- Oban ale (anag)" , "Edible mollusc - on a bale (anag)" , "Mollusc, the sea-ear" .)