Excellent youngster getting over self-defence (6)
I believe the answer is:
aikido
'self-defence' is the definition.
(aikido is a kind of self-defence)
'excellent youngster getting over' is the wordplay.
I cannot really see how this works, but
'excellent' could be 'ai' (resembles 'A1', old-fashioned term for excellent) and 'ai' is found within the answer.
'youngster' could be 'kid' (synonyms) and 'kid' is present in the answer.
'over' could be 'o' (cricket abbreviation) and 'o' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for aikido that I've seen before include "Defensive moves" , "Martial art, akin to judo" , "Japanese form of self-defence" , "Japanese martial art similar to judo" , "a particular form of art" .)