King follows period in office beset by enemy alien (9)
I believe the answer is:
foreigner
'alien' is the definition.
(synonyms)
'king follows period in office beset by enemy' is the wordplay.
'king' becomes 'r' (r is abbreviation for rex).
'follows' is a charade indicator (letters next to each other).
'period in office' becomes 'reign' (eg the reign or tenure of a prime minister).
'beset by' indicates putting letters inside ('beset' could historically mean 'cover').
'enemy' becomes 'foe' (I've seen this before).
'reign' put into 'foe' is 'foreigne'.
'r' put after 'foreigne' is 'FOREIGNER'.
(Other definitions for foreigner that I've seen before include "asylum seeker?" , "Stranger, outsider" , "Alien" , "chauvinist holds in contempt" , "One from abroad" .)