I failed to pick up that energy-packed cereal (5)
I believe the answer is:
wheat
'cereal' is the definition.
(I know that wheat is a type of cereal)
'i failed to pick up that energy-packed' is the wordplay.
'i failed to pick up that' becomes 'what' (I didn't catch that).
'energy' becomes 'e' (physics symbol as in E=mc2).
'packed' indicates putting letters inside (some letters are packed into others).
'what' enclosing 'e' is 'WHEAT'.
(Other definitions for wheat that I've seen before include "Cereal grass" , "Type of cereal" , "Cereal grain" , "Cereal crop" , "Hat we turn into grain" .)