Martial art – weak art? Not weak, quite wrong (6)
I believe the answer is:
karate
'martial art' is the definition.
(karate is a kind of martial art)
'weak art? not weak quite wrong' is the wordplay.
'not' is a deletion indicator.
'weak' becomes 'w' (physics symbol as in W-boson).
'quite wrong' is an anagram indicator.
'weak'+'art'='weakart'
'weakart' with 'w' taken away is 'eakart'.
'eakart' with letters rearranged gives 'KARATE'.
(Other definitions for karate that I've seen before include "A taker (anag)" , "Japanese art of unarmed combat" , "fighting system" , "Japanese system of unarmed combat" , "Unarmed-combat system" .)