Greek character, priest, consuming small cereal (6)
I believe the answer is:
muesli
'cereal' is the definition.
(I know this)
'greek character priest consuming small' is the wordplay.
'greek character' becomes 'mu' (letter from the Greek alphabet).
'priest' becomes 'eli' (biblical figure).
'consuming' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'eli' enclosing 's' is 'esli'.
'mu'+'esli'='MUESLI'
(Other definitions for muesli that I've seen before include "Granola-like food" , "Breakfast cereal mix" , "Breakfast food of cereal, nuts, dried fruit etc with milk" , "Breakfast food of cereals, fruits, nuts etc" , "Swiss breakfast?" .)