Mount that is dangerous when active (7)

I believe the answer is:

volcano

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'mount' is the definition.
(I know that volcano is a type of mount)

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

Can you help me to learn more?

(Other definitions for volcano that I've seen before include "Exploding mountain" , "St Helens?" , "A fissure in the earth's crust with molten rock" , "eg Mount Etna" , "One expelling lava etc." .)

I've seen this clue in The Globe and Mail.

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