Martial art in which thousand charge around area (6)
I believe the answer is:
karate
'martial art' is the definition.
(karate is a kind of martial art)
'thousand charge around area' is the wordplay.
'thousand' becomes 'k' ('K' can be a synonym of 'thousand').
'charge' becomes 'rate' (rate is a kind of charge**).
'around' indicates putting letters inside.
'area' becomes 'a' (maths abbreviation).
'k'+'rate'='krate'
'krate' enclosing 'a' is 'KARATE'.
'in which' acts as a link.
(Other definitions for karate that I've seen before include "Jap. combat; sort of chop" , "Japanese system of unarmed combat" , "Japanese martial art" , "Unarmed-combat system" , "Japanese art of unarmed combat" .)