West Indian hostile to assassin casing area (8)
I believe the answer is:
antiguan
'west indian' is the definition.
(Antiguan is an example)
'hostile to assassin casing area' is the wordplay.
'hostile to' becomes 'anti' (anti means hostile to or opposed to).
'assassin' becomes 'gun' (as in a hired gun).
'casing' is an insertion indicator (I've seen this in other clues).
'area' becomes 'a' (maths abbreviation).
'gun' enclosing 'a' is 'guan'.
'anti'+'guan'='ANTIGUAN'
(Other definitions for antiguan that I've seen before include "from the West Indies" , "Islander" , "From a Caribbean island paired with Barbuda" , "West Indian" .)