Crossword Genius

At sea, no greater supplier of energy (9)

Ross

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" .)

I've seen this clue in The Telegraph.
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