It's hell for nine at sea (7)
I believe the answer is:
inferno
'it's 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' with letters rearranged gives 'INFERNO'.
(Other definitions for inferno that I've seen before include "blazer?" , "Fierce fire" , "Dante's Hell" , "Where Satan lives and sinners go" , "Large uncontrolled fire" .)