In Hogwarts, a new sickbay (3)
I believe the answer is:
san
'sickbay' is the definition.
(I've seen this before)
'in hogwarts a new' is the wordplay.
'in' says the answer is hidden in the clue.
'SAN' is hidden within 'hogwarts a new'.
(Other definitions for san that I've seen before include "School medical facility" , "Health resort" , "Japanese title" , "Hospital (abbrev.)" , "- - Francisco" .)