South African man-of-war (4)
I believe the answer is:
boer
'south african' is the definition.
(Dutch settler in South Africa)
'man-of-war' is the wordplay.
'man' becomes 'Bo'.
'of-war' becomes 'er' (I am not sure about this - if you are sure you should believe this answer much more).
'bo'+'er'='BOER'
Can you help me to learn more?
(Other definitions for boer that I've seen before include "White South African of Dutch descent" , "once our enemy" , "Afrikaner" , "One we fought in 1900" , "South African of Dutch ancestry" .)