Moving mountains with no loss of movement in the ocean (7)

I believe the answer is:

tsunami

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

'movement in the ocean' is the definition.
'tsunami' can be an answer for 'movement' (tsunami is a kind of movement). I am not sure about the 'in the ocean' bit.

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for tsunami that I've seen before include "Freak wave" , "Great, fast-moving sea wave" , "Large sea wave caused by an earthquake" , "I am nuts! (anag)" , "Strong surge" .)

I've seen this clue in the Sydney Morning Herald.

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