It sparks off a megaton explosion (7)

I believe the answer is:

magneto

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'it sparks off a' is the definition.
The definition suggests a singular noun which matches the answer.

'megaton explosion' is the wordplay.
'explosion' indicates an anagram.
'megaton' is an anagram of 'MAGNETO'.

Can you help me to learn more?

(Other definitions for magneto that I've seen before include "generation device" , "Car generator" , "Ignition device" , "Small type of generator" , "Small electric generator" .)

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

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