The part of Mephistopheles is not for her! (3)
I believe the answer is:
his
'her' is the definition.
I can't tell whether this defines the answer.
'the part of mephistopheles is not' is the wordplay.
'the part of' says the answer is hidden in the clue.
'HIS' can be found hidden inside 'mephistopheles not'.
'for' is the link.
Can you help me to learn more?
(Other definitions for his that I've seen before include "To - Coy Mistress (Marvell)" , "Belonging to that man" , "that chap's" , "That man's" , "Belonging to the male" .)