The best stuff is packed with energy (5)
I believe the answer is:
cream
'the best' is the definition.
(as in 'the cream of the crop')
'stuff is packed with energy' is the wordplay.
'stuff' becomes 'cram' (synonyms).
'is packed with' is an insertion indicator.
'energy' becomes 'e' (physics symbol as in E=mc2).
'cram' going around 'e' is 'CREAM'.
(Other definitions for cream that I've seen before include "An off-white shade" , "Pick - fatty part of milk" , "'There are more ways of killing a cat than choking it with ....." , "Elite; off-white" , "Beat" .)