Makes boxes to contain energy (7)
I believe the answer is:
creates
'makes' is the definition.
('create' can be a synonym of 'make')
'boxes to contain energy' is the wordplay.
'boxes' becomes 'crates' (crate is a kind of box).
'to contain' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'crates' enclosing 'e' is 'CREATES'.
(Other definitions for creates that I've seen before include "Produces" , "coins" , "Complains" , "Makes" , "Generates" .)