Hell for nine at sea (7)
I believe the answer is:
inferno
'hell' is the definition.
(inferno can mean hell as in Dante's Inferno)
'for nine at sea' is the wordplay.
'at sea' indicates anagramming the letters.
'for'+'nine'='fornine'
'fornine' anagrammed gives 'INFERNO'.
(Other definitions for inferno that I've seen before include "Uncontrollable fire" , "(Fiery) Hell" , "Intense and uncontrolled fire" , "Hellish conflagration" , "A raging fire like hell" .)