Deadly substance in fatal avalanche (4)

I believe the answer is:

lava

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

'deadly substance' is the definition.
'lava' can be an answer for 'substance' (lava is a kind of substance). I'm not sure about the 'deadly' bit.

'in fatal avalanche' is the wordplay.
'in' indicates the answer is hidden within the clue.
'LAVA' is hidden within 'fatal avalanche'.

Can you help me to learn more?

(Other definitions for lava that I've seen before include "Hot molten rock" , "What volcano ejects" , "Volcanic stuff" , "Flowing volcanic matter" , "Molten volcanic material" .)

I've seen this clue in The Daily Express.

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