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Eel squirms in bag on naturalist's vessel. (6)

Ross

I believe the answer is:

beagle

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This might be a double definition.

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

'squirms in bag on naturalist's vessel' is the second definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

Can you help me to learn more?

(Other definitions for beagle that I've seen before include "Hound dog" , "5 [MANS BEST FRIEND]" , "hunting animal with smooth coat" , "Small hunting dog" , "Small, sturdy breed of hound" .)

I've seen this clue in The Irish Times.
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