Starts jumping in unmitigated joy. It's beginning to upset self-defence system (8)
I believe the answer is:
jiujitsu
'self-defence system' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'starts jumping in unmitigated joy it's beginning to upset' is the wordplay.
'starts' says to take the initial letters.
'beginning to' indicates taking the first letters.
The initial letters of 'jumping in unmitigated joy' is 'jiuj'.
The initial letter of 'upset' is 'u'.
'jiuj'+'its'+'u'='JIUJITSU'
Can you help me to learn more?
(Other definitions for jiujitsu that I've seen before include "Japanese system of unarmed combat" , "Martial art" , "Self-defence method without weapons" .)