Sage Derby, say, eaten by parent (7)
I believe the answer is:
mahatma
'sage' is the definition.
(term for a wise Hindu leader)
'derby say eaten by parent' is the wordplay.
'derby say' becomes 'hat' (derby is a type of hat).
'eaten by' means one lot of letters goes inside another.
'parent' becomes 'mama' (mama is a kind of parent).
'hat' put into 'mama' is 'MAHATMA'.
(Other definitions for mahatma that I've seen before include "venerable soul" , "Gandhi's title" , "Holy man" , "Wise and holy leader in 12" , "Savant" .)