Sebastian and Sean upset English people of South American republic (8)

I believe the answer is:

guyanese

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'people of south american republic' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.

'sebastian and sean upset english' is the wordplay.
'sebastian' becomes 'guy' (I can't explain this - if you can you should give a lot more credence to this answer).
'and' is a charade indicator (letters next to each other).
'upset' is an anagram indicator.
'english' becomes 'e' (abbreviation).
'sean' with letters rearranged gives 'anes'.
'guy'+'anes'+'e'='GUYANESE'

Can you help me to learn more?

(Other definitions for guyanese that I've seen before include "from South America" , "From a South American country, capital Georgetown" .)

I've seen this clue in the Sydney Morning Herald.

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