Best stuff contains energy (5)
I believe the answer is:
cream
'best' is the definition.
(thesaurus)
'stuff contains energy' is the wordplay.
'stuff' becomes 'cram' (cramming is a kind of stuffing**).
'contains' is an insertion indicator.
'energy' becomes 'e' (physics symbol as in E=mcr).
'cram' placed around 'e' is 'CREAM'.
(Other definitions for cream that I've seen before include "best people" , "'There are more ways of killing a cat than choking it with ....." , "Best - substitute for shaving soap" , "treat for cat?" , "Beat" .)