Foreigner called to interrupt rest, unfortunately (8)
I believe the answer is:
stranger
'foreigner' is the definition.
(thesaurus)
'called to interrupt rest unfortunately' is the wordplay.
'called' becomes 'rang' ('ring' can be a synonym of 'call').
'to interrupt' means one lot of letters goes inside another.
'unfortunately' indicates an anagram.
'rest' anagrammed gives 'ster'.
'rang' put inside 'ster' is 'STRANGER'.
(Other definitions for stranger that I've seen before include "Rent rags (anag)" , "One who is little known" , "visitor" , "'Weirder, odder (8)'" , "unfamiliar figure" .)