Excellent knockout I'd welcomed in self-defence (6)
I believe the answer is:
aikido
'self-defence' is the definition.
(aikido is a kind of self-defence)
'excellent knockout i'd welcomed' is the wordplay.
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'knockout' becomes 'ko' (abbreviation).
'welcomed' means one lot of letters goes inside another (inserted letters are welcomed in).
'ko' going around 'id' is 'kido'.
'ai'+'kido'='AIKIDO'
'in' is the link.
(Other definitions for aikido that I've seen before include "Martial art, akin to judo" , "Japanese form of self-defence" , "a particular form of art" , "Defensive moves" , "Japanese martial art similar to judo" .)