One child in a ring displaying martial art (6)
I believe the answer is:
aikido
'martial art' is the definition.
(aikido is a kind of martial art)
'one child in a ring' is the wordplay.
'one' becomes 'i' (Roman numeral).
'child' becomes 'kid' ('kid' can be a synonym of 'child'**).
'in' indicates putting letters inside.
'ring' becomes 'o'.
'i'+'kid'='ikid'
'a'+'o'='ao'
'ikid' put inside 'ao' is 'AIKIDO'.
'displaying' is the link.
(Other definitions for aikido that I've seen before include "Defensive moves" , "a particular form of art" , "Japanese form of self-defence" , "one of the martial arts" , "Martial art, akin to judo" .)