Martial art Ken attempted initially with speed (6)
I believe the answer is:
karate
'martial art' is the definition.
(karate is a kind of martial art)
'ken attempted initially with speed' is the wordplay.
'initially' says to take the initial letters.
'with' means one lot of letters go next to another.
'speed' becomes 'rate' (the speed at which something happens).
The first letters of 'ken attempted' is 'ka'.
'ka'+'rate'='KARATE'
(Other definitions for karate that I've seen before include "A Jap. martial art" , "Oriental-style chops" , "A taker (anag)" , "Japanese form of unarmed combat" , "fighting system" .)