Immigrant officer shot a lion (8)
I believe the answer is:
colonial
'immigrant' is the definition.
(thesaurus)
'officer shot a lion' is the wordplay.
'officer' becomes 'col' (short for colonel).
'shot' indicates anagramming the letters.
'a'+'lion'='alion'
'alion' with letters rearranged gives 'onial'.
'col'+'onial'='COLONIAL'
(Other definitions for colonial that I've seen before include "Like America before independence" , "Settler" , "Maybe overseas" , "Refers to territorial possessions controlled by a ruling state" , "Relating to overseas dominions" .)