Fools back in diocese, egotistical (5)
I believe the answer is:
geese
'fools' is the definition.
(goose can mean a foolish person)
'back in diocese egotistical' is the wordplay.
'back' shows that the letters should be reversed in order.
'in' says the answer is hidden in the clue.
'eseeg' is hidden within 'diocese egotistical'.
'eseeg' backwards is 'GEESE'.
(Other definitions for geese that I've seen before include "Water birds; fools" , "Birds that fly in a skein" , "Protectors of city" , "Large water birds" , "wingers" .)