Trees flourished, it's said, in leading industrial nations (5,5)

I believe the answer is:

first world

'leading industrial nations' is the definition.
(first world nations)

'trees flourished it's said' is the wordplay.
'trees' becomes 'firs' (fir is a kind of tree).
'flourished' becomes 'twirled'.
'it's said' shows a homophone (sound like).
'twirled' is a homophone of 'tworld'.
'firs'+'tworld'='FIRST WORLD'

'in' acts as a link.

I've seen this clue in The Guardian.

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