Food energy in clear mixture (6)
I believe the answer is:
cereal
'food' is the definition.
(cereal is a kind of food)
'energy in clear mixture' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'in' is an insertion indicator.
'mixture' indicates an anagram.
'clear' anagrammed gives 'ceral'.
'e' placed within 'ceral' is 'CEREAL'.
(Other definitions for cereal that I've seen before include "11 food, maybe" , "Breakfast food made from grain" , "Contents of breakfast bowl" , "Food prepared from grain" , "Eg, wheat, rye, corn" .)