Say Harry Potter is jolly good! (6)
I believe the answer is:
wizard
'jolly good' is the definition.
(I've seen this before)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for wizard that I've seen before include "A magician, perhaps from Oz" , "Magician, necromancer" , "Fantastic" , "Superb" , "Warlock" .)