He’s good at karate (3)
I believe the answer is:
'he's' is the definition.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for dan that I've seen before include "book (abbreviated version)" , "Karate expert" , "Skill level in judo" , "-- Dare, comic hero" , "Man's name; judo grade" .)