Sorcerer, one close to Oz, enters part of hospital (6)
I believe the answer is:
wizard
'sorcerer' is the definition.
(synonyms)
'one close to oz enters part of hospital' is the wordplay.
'one' becomes 'i' (Roman numeral).
'close to' indicates one should take the final letters (the 'close' to the word).
'enters' indicates putting letters inside.
'part of hospital' becomes 'ward' (hospitals have wards).
The last letter of 'oz' is 'z'.
'i'+'z'='iz'
'iz' going within 'ward' is 'WIZARD'.
(Other definitions for wizard that I've seen before include "A magician, perhaps from Oz" , "Hotshot" , "Genius" , "Hogwarts' graduate - Super!" , "Jolly good" .)