Unwilling republican intervenes between countries (10)
I believe the answer is:
interstate
'between countries' is the definition.
'unwilling republican intervenes' is the wordplay.
'unwilling' becomes 'intestate' (I've seen this before).
'republican' becomes 'r' (abbreviation).
'intervenes' means one lot of letters goes inside another.
'intestate' enclosing 'r' is 'INTERSTATE'.
(Other definitions for interstate that I've seen before include "road in US" , "American motorway" , "highway" , "between parts of US" , "American way" .)