The best stuff, full of energy (5)
I believe the answer is:
cream
'the best' is the definition.
(I've seen this before)
'stuff full of energy' is the wordplay.
'stuff' becomes 'cram' (cramming is a kind of stuffing**).
'full of' is an insertion indicator.
'energy' becomes 'e' (physics symbol as in E=mcr).
'cram' going around 'e' is 'CREAM'.
(Other definitions for cream that I've seen before include "Very best; pale yellow" , "Elite; colour" , "The best part -of the crop" , "Dairy product" , "Butterfat part of milk" .)