What made the plane burst into flames? (6,4)

I believe the answer is:

forest fire

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

'what made' is the definition.
I can't judge whether this definition defines the answer.

'plane burst into flames?' is the wordplay.
'plane' becomes 'fore plane' (fore plane is a kind of plane).
'burst into' becomes 'st' (I can't explain this - if you can you should give a lot more credence to this answer).
'flames?' becomes 'fire' (I've seen this in another clue).
'fore'+'st'+'fire'='FOREST FIRE'

'the' acts as a link.

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