Property of Mephistopheles (3)
I believe the answer is:
his
'property' is the definition.
Although both the answer and definition are nouns, they are not in the same form of the noun.
'of mephistopheles' is the wordplay.
'of' indicates a hidden word.
'HIS' is hidden in the letters of 'mephistopheles'.
Can you help me to learn more?
(Other definitions for his that I've seen before include "Belonging to the male" , "That man's" , "Belonging to that man" , "It belongs to him" , "To - Coy Mistress (Marvell)" .)