Minimum energy during final (5)
I believe the answer is:
least
'minimum' is the definition.
(similar in meaning)
'energy during final' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'during' means one lot of letters goes inside another.
'final' becomes 'last' (synonyms).
'e' going within 'last' is 'LEAST'.
(Other definitions for least that I've seen before include "Of minimal significance" , "Smallest in amount or degree" , "Minimal amount" , "Smallest, slightest" , "Opposite of most" .)