Fantastic takedown grabbing head of opponent in 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)
'fantastic takedown grabbing head of opponent' is the wordplay.
'fantastic' indicates anagramming the letters.
'grabbing' is an insertion indicator.
'head of' suggests taking the first letters.
The first letter of 'opponent' is 'o'.
'takedown' with letters rearranged gives 'taekwndo'.
'taekwndo' placed around 'o' is 'TAE KWON DO'.
'in' acts as a link.
(Other definitions for tae kwon do that I've seen before include "sport" , "combat" , "Martial art Olympic event since 2000" , "Martial art display" , "Modern Korean martial art" .)