Plant in main island almost meeting fuel total (9)

I believe the answer is:

sargassum

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'plant in main island' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

'almost meeting fuel total' is the wordplay.
'almost' becomes 'sar' (I can't explain this - if you can you should give a lot more credence to this answer).
'meeting' says to put letters next to each other.
'fuel' becomes 'gas' (gas is a fuel).
'total' becomes 'sum' (synonyms).
'sar'+'gas'+'sum'='SARGASSUM'

Can you help me to learn more?

(Another definition for sargassum that I've seen is " seaweed".)

I've seen this clue in The Times.

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