Unstable enemy country (5)
I believe the answer is:
yemen
'country' is the definition.
(I know that Yemen is a country)
'unstable enemy' is the wordplay.
'unstable' indicates anagramming the letters.
'enemy' is an anagram of 'YEMEN'.
(Other definitions for yemen that I've seen before include "Country to W. of Oman" , "Republic by the Red Sea" , "Country bordering Oman" , "Saudi neighbour" , "nation" .)