Starts off hostilities, a Kiwi activity (4)
I believe the answer is:
haka
'starts off hostilities a kiwi activity' is the definition.
I don't know anything about this answer so I can't tell whether this works.
'starts off hostilities a kiwi activity' is the wordplay.
'starts off' indicates taking the first letters.
'kiwi activity' becomes 'ka' (this could be a standard abbreviation of which I'm unaware).
The initial letter of 'hostilities' is 'h'.
'h'+'a'+'ka'='HAKA'
(Other definitions for haka that I've seen before include "Aggressive Kiwi dance before a rugby match" , "intimidatory chant" , "pre-match ritual" , "Maori ceremonial war dance" , "routine before start of match" .)