Crossword Genius

How cameraman prepares apparatus for big explosion (5,2,6)

Ross

I believe the answer is:

mount st helens

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

'apparatus for big explosion' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'how cameraman prepares' is the wordplay.
I cannot really understand how this works, but
'prepares' could be 'mounts' (mounting is a kind of preparing) and 'mounts' is found within the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in The Independent.
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