The best stuff conserves energy (5)
I believe the answer is:
cream
'the best' is the definition.
(as in 'the cream of the crop')
'stuff conserves energy' is the wordplay.
I cannot quite see how this works, but
'energy' could be 'e' (physics symbol as in E=mc2) and 'e' is located in the answer.
'stuff' could be 'cram' (cramming is a kind of stuffing) and 'cram' is found within the leftover letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for cream that I've seen before include "The best part -of the crop" , "Fat-rich part of milk" , "Beat" , "Milk product" , "'There are more ways of killing a cat than choking it with ....." .)