Grain, that which is packed with energy (5)
I believe the answer is:
wheat
'grain' is the definition.
(I know that wheat is a type of grain)
'which is packed with energy' is the wordplay.
'which' becomes 'what' (I've seen this in other clues).
'is packed with' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'what' enclosing 'e' is 'WHEAT'.
'that' acts as a link.
(Other definitions for wheat that I've seen before include "Grass whose grain is used to make flour" , "Tall grass producing light brown grain" , "(Pasta) cereal" , "Field crop" , "Annual grass with light brown grains" .)