Top stuff contains energy (5)
I believe the answer is:
cream
'top' 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 "An off-white shade" , "Beat" , "The best part -of the crop" , "Best - substitute for shaving soap" , "Dairy product" .)