Catherine shackles artist in sport (6)
I believe the answer is:
karate
'sport' is the definition.
(I have seen 'Combative sport ' mean 'karate' so perhaps 'sport' could also mean 'karate')
'catherine shackles artist' is the wordplay.
'catherine' becomes 'kate'.
'shackles' means one lot of letters goes inside another (shackle can mean imprison).
'artist' becomes 'ra' (Royal Academician, member of Royal Academy of Arts).
'kate' enclosing 'ra' is 'KARATE'.
'in' acts as a link.
(Other definitions for karate that I've seen before include "Japanese art of unarmed combat" , "A taker (anag)" , "Japanese system of unarmed combat" , "Combative sport" , "Martial art (with chopping?)" .)