Minimal energy required in final (5)
I believe the answer is:
least
'minimal' is the definition.
(similar in meaning)
'energy required in final' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'required in' is an insertion indicator.
'final' becomes 'last' (synonyms).
'e' placed within 'last' is 'LEAST'.
(Other definitions for least that I've seen before include "Smallest in amount or degree" , "Fewest" , "Smallest (in significance)" , "Smallest, slightest" , "No more petty" .)