The best stuff to conserve energy (5)
I believe the answer is:
cream
'the best' is the definition.
(as in 'the cream of the crop')
'stuff to conserve energy' is the wordplay.
'stuff' becomes 'cram' (cramming is a kind of stuffing).
'to conserve' indicates putting letters inside.
'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 "Milk product" , "Butterfat part of milk" , "Top of milk or society" , "Best - substitute for shaving soap" , "Best - ointment" .)