Opening of hostilities, also known as the rugby challenge (4)
I believe the answer is:
haka
'rugby challenge' is the definition.
I know nothing about this answer so I can't tell whether this works.
'opening of hostilities also known as' is the wordplay.
'opening of' says to take the initial letters.
'also known as' becomes 'a.k.a.' (synonyms).
The initial letter of 'hostilities' is 'h'.
'h'+'aka'='HAKA'
'the' is the link.
Can you help me to learn more?
(Other definitions for haka that I've seen before include "pre-match ritual" , "routine before start of match" , "intimidatory chant" , "Aggressive Kiwi dance before a rugby match" , "Maori ceremonial war dance" .)