Part of Prospero, leading part (4)
I believe the answer is:
role
'part' is the definition.
(eg a theatrical role)
'of prospero leading part' is the wordplay.
'of' says the answer is hidden in the clue.
'ROLE' is hidden in the letters of 'prospero leading part'.
(Other definitions for role that I've seen before include "Part played (in life?)" , "Part, portrayal" , "Function in life" , "Function; part" , "Playing part" .)