Fire allowed to spread (5)

I believe the answer is:

letgo

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'fire' is the definition.
I know nothing about this answer so I can't judge whether this works.

'allowed to spread' is the wordplay.
'allowed' becomes 'let' (to let is to allow or permit).
'to spread' becomes 'go' (I can't explain this - if you can you should believe this answer much more).
'let'+'go'='LETGO'

Can you help me to learn more?

I've seen this clue in The Daily Express.

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