Ian came unexpectedly with Republican statesman (8)
I believe the answer is:
american
'statesman' is the definition.
(I've seen this before)
'ian came unexpectedly with republican' is the wordplay.
'unexpectedly with' indicates anagramming the letters.
'republican' becomes 'r' (abbreviation).
'ian'+'came'='iancame'
'iancame' with its letters mixed up with 'r' can make 'AMERICAN'.
(Other definitions for american that I've seen before include "Beauty of Film" , "One from the States" , "Film starring Kevin Spacey" , "US-born" , "statesman" .)