Exceptional youngster facing old martial art (6)
I believe the answer is:
aikido
'martial art' is the definition.
(aikido is a kind of martial art)
'exceptional youngster facing old' is the wordplay.
'exceptional' becomes 'AI' (resembles Ak, informal term meaning excellent).
'youngster' becomes 'kid' (synonyms).
'facing' means one lot of letters go next to another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'ai'+'kid'+'o'='AIKIDO'
(Other definitions for aikido that I've seen before include "Martial art, akin to judo" , "Japanese martial art similar to judo" , "Japanese form of self-defence" , "Defensive moves" , "a particular form of art" .)