Poseidon goes off, holding a key (6)

I believe the answer is:

seagod

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'poseidon' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'goes off holding a key' is the wordplay.
'off' indicates an anagram.
'holding' is an insertion indicator.
'key' becomes 'd' (musical key).
'goes' anagrammed gives 'sego'.
'sego' enclosing 'a' is 'seago'.
'seago'+'d'='SEAGOD'

Can you help me to learn more?

(Another definition for seagod that I've seen is " marine commander".)

I've seen this clue in The Daily Express.

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