Sage Derby perhaps eaten by mum twice (7)
I believe the answer is:
mahatma
'sage' is the definition.
(term for a wise Hindu leader)
'derby perhaps eaten by mum twice' is the wordplay.
'derby perhaps' becomes 'hat' (derby is a type of hat).
'eaten by' indicates putting letters inside (inserted letters get eaten).
'mum' becomes 'ma' (ma can mean mother).
'twice' indicates the doubling of some letters.
'ma' duplicated is 'mama'.
'hat' going inside 'mama' is 'MAHATMA'.
(Other definitions for mahatma that I've seen before include "Gandhi, for example" , "Holy person or sage in South Asia" , "Hindu leader" , "Gandhi's title" , "(Indian) holy person or sage" .)