Sage Derby perhaps eaten by mummy? (7)
I believe the answer is:
mahatma
'sage' is the definition.
(term for a wise Hindu leader)
'derby perhaps eaten by mummy?' is the wordplay.
'derby perhaps' becomes 'hat' (derby is a type of hat).
'eaten by' is an insertion indicator.
'mummy?' becomes 'mama' ('mama' can be a synonym of 'mummy').
'hat' inserted within 'mama' is 'MAHATMA'.
(Other definitions for mahatma that I've seen before include "In India, a title of respect as for Gandhi" , "Gandhi, for example" , "Holy man" , "Sage (as e.g. Gandhi)" , "Wise and holy leader in 12" .)