Source of the flame that caused the forest fire? (6,4)

'the flame that caused the forest fire?' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

'source' is the wordplay.
I cannot quite understand how this works, but
'source' could be 'tree' (I have seen 'Source of wood ' mean 'tree' so perhaps 'source' could also mean 'tree') and 'tree' is present in the answer.
The remaining letters 'dragon' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).

'of' acts as a link.

Can you help me to learn more?