Terror of the deep (3,7)

I think it could be either:

Which length description are you looking for?:

3,7

10*

sea monster

'terror of the deep' is the definition.
(I've seen this in another clue)

This is the entire clue.

(Other definitions for sea monster that I've seen before include "fabulous swimmer" , "dangerous creature in the main" , "A danger to mariners" .)

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