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' (cramming is a kind of stuffing).
'is packed with' is an insertion indicator.
'energy' becomes 'e' (physics symbol as in E=mcr).
'cram' enclosing '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" .)