At sea, no greater supplier of energy (9)
I believe the answer is:
generator
'supplier of energy' is the definition.
'at sea no greater' is the wordplay.
'at sea' indicates anagramming the letters.
'no'+'greater'='nogreater'
'nogreater' with letters rearranged gives 'GENERATOR'.
(Other definitions for generator that I've seen before include "Device creating electrical energy" , "Engine to produce electricity" , "Creator" , "Eg, dynamo" , "juice maker" .)