Trained at kendo fencing wife -- old martial art (3,4,2)
I believe the answer is:
tae kwon do
'martial art' is the definition.
(tae kwon do is a kind of martial art)
'trained at kendo fencing wife old' is the wordplay.
'trained' indicates an anagram (to train can mean to grow something into a shape).
'fencing' is an insertion indicator (to fence can mean to contain or put a limit round).
'wife' becomes 'w'.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'at'+'kendo'='atkendo'
'atkendo' anagrammed gives 'taekndo'.
'w'+'o'='wo'
'taekndo' going around 'wo' is 'TAE KWON DO'.
(Other definitions for tae kwon do that I've seen before include "Martial art Olympic event since 2000" , "Koreans fighting" , "Martial art display" , "Modern Korean martial art" , "sport" .)