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

I believe the answer is:
byelorussian
'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'.
Can you help me to learn more?
I've seen this clue in The Times.
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