Naval hero, but a beast (3-4)

I believe the answer is:

sea-lion

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'a beast' is the definition.
The answer and definition can be both animals as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

'naval hero' is the wordplay.
'naval' becomes 'sea' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'hero' becomes 'lion' (lion can mean a brave person).
'sea'+'lion'='SEA-LION'

'but' is the link.

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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