Rains and is most upsetting to foreigners (10)

'foreigners' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.

'rains and is most upsetting to' is the wordplay.
'most' means to remove the last letter.
'upsetting' indicates anagramming the letters.
'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 taken off is 'rainsandi'.
'rainsandi' with letters rearranged gives 'sardinian'.
'sardinian'+'s'='SARDINIANS'

Can you help me to learn more?