Minimum energy put in at the end (5)
I believe the answer is:
least
'minimum' is the definition.
(similar in meaning)
'energy put in at the end' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'put in' is an insertion indicator (back).
'at the end' becomes 'last' (I've seen this in another clue).
'e' going within 'last' is 'LEAST'.
(Other definitions for least that I've seen before include "Lowest or smallest" , "Being lowest in importance - gone stale" , "To the lowest degree" , "Bottom" , "superlatively small" .)