Old European's say-so left Brunei reeling (12)

I believe the answer is:

byelorussian

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'old european's' is the definition.
'Byelorussian' can be an answer for 'european's' (Byelorussian is an example). I am not certain of the 'old' bit.

'say-so left brunei reeling' is the wordplay.
'left' becomes 'l' (common abbreviation).
'reeling' indicates anagramming the letters.
'say'+'so'+'l'+'brunei'='saysolbrunei'
'saysolbrunei' with letters rearranged gives 'BYELORUSSIAN'.

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I've seen this clue in The Times.

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