Make box to contain energy (6)
I believe the answer is:
create
'make' is the definition.
(I know that make is a more specific form of create)
'box to contain energy' is the wordplay.
I cannot really understand how this works, but
'energy' could be 'e' (physics symbol as in E=mc2) and 'e' is found in the answer.
'box' could be 'crate' (crate is a kind of box) and 'crate' is located in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for create that I've seen before include "Make, Porduce" , "Design" , "make a scene" , "Concoct" , "coin" .)