Fixed way to conserve energy (3)
I believe the answer is:
set
'fixed' is the definition.
(both can mean decided or determined)
'way to conserve energy' is the wordplay.
'way' becomes 'st' (abbreviation for street).
'to conserve' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mc2).
'st' placed around 'e' is 'SET'.
(Other definitions for set that I've seen before include "Gel; start (a fire)" , "Put down the tennis group" , "become firm" , "Complete collection" , "Harden; fixed; group" .)