Food energy in clear liquid (6)
I believe the answer is:
cereal
'food' is the definition.
(I know that cereal is a type of breakfast food)
'energy in clear liquid' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'in' is an insertion indicator.
'liquid' indicates an anagram.
'clear' is an anagram of 'creal'.
'e' going within 'creal' is 'CEREAL'.
(Other definitions for cereal that I've seen before include "Grain crop" , "Edible grain for breakfast" , "Breakfast dish" , "Grain used for food" , "'Wheat or maize, say (6)'" .)