Island politician primarily investigating old African warriors (4)
I believe the answer is:
impi
'old african warriors' is the definition.
(term for a group of Zulu warriors)
'island politician primarily investigating' is the wordplay.
'island' becomes 'i'.
'politician' becomes 'MP' (Member of Parliament).
'primarily' indicates taking the first letters.
The first letter of 'investigating' is 'i'.
'i'+'mp'+'i'='IMPI'
(Other definitions for impi that I've seen before include "Regiment of Zulu warriors" , "Group of Bantu warriors" , "group of fighting men" , "native warriors?" , "Band of Zulu warriors" .)