Rains and is most upsetting to foreigners (10)

'foreigners' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'rains and is most upsetting to' is the wordplay.
'most' means to remove the last letter.
'upsetting' indicates an anagram.
'to' becomes 's' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'rains'+'and'+'is'='rainsandis'
'rainsandis' with its last letter removed is 'rainsandi'.
'rainsandi' is an anagram of 'sardinian'.
'sardinian'+'s'='SARDINIANS'

Can you help me to learn more?