Submarine controlled by steward leaving island (10)

I believe the answer is:

underwater

I'm a little stuck... Click here to teach me more about this clue!

'submarine' is the definition.
(thesaurus)

'controlled by steward leaving island' is the wordplay.
I cannot quite see how this works, but
'by' could be 'at' and 'at' is found within the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Another definition for underwater that I've seen is " Submerged".)

I've seen this clue in The Times and The Sunday Times.

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