Sailor, solo with a marine mollusc (7)
I believe the answer is:
abalone
'marine mollusc' is the definition.
(I've seen this before)
'sailor solo with a' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'solo' becomes 'lone' (both can mean alone).
'with' is a charade indicator (letters next to each other).
'lone' put after 'a' is 'alone'.
'ab'+'alone'='ABALONE'
(Other definitions for abalone that I've seen before include "One lives in a shell" , "Edible marine mollusc -- Oban ale (anag)" , "Source of mother-of-pearl" , "Would you be able on a US shellfish?" , "Edible shellfish, sea-ear" .)