Sat in on exchange one diplomatically joining the institution that is United? (7)

I believe the answer is:

nations

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'one diplomatically joining the institution that is united?' is the definition.
Although both the answer and 'one' are nouns, they are not in the same form of the noun.

'sat in on exchange' is the wordplay.
'exchange' indicates an anagram.
'sat'+'in'+'on'='satinon'
'satinon' is an anagram of 'NATIONS'.

Can you help me to learn more?

(Other definitions for nations that I've seen before include "Countries" , "No stain (anag.)" .)

I've seen this clue in the Irish Times and the Irish Times (Crosaire).

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