Seafood (and nothing else) sustains sailor (7)
I believe the answer is:
abalone
'seafood' is the definition.
(type of sea snail)
'and nothing else sustains sailor' is the wordplay.
'and nothing else' becomes 'alone'.
'sustains' means one lot of letters go next to another.
'sailor' becomes 'ab' (abbreviation for able seaman).
'alone' put after 'ab' is 'ABALONE'.
(Other definitions for abalone that I've seen before include "Sea food must be found" , "produces an ear shell" , "marine creature" , "paua" , "Edible mollusc - on a bale (anag)" .)