Last under failing energy (6)
I believe the answer is:
endure
'last' is the definition.
(I know that last can be written as endure)
'under failing energy' is the wordplay.
'failing' is an anagram indicator (a failed spelling of the word).
'energy' becomes 'e' (physics symbol as in E=mc2).
'under' anagrammed gives 'endur'.
'endur'+'e'='ENDURE'
(Other definitions for endure that I've seen before include "Last or undergo" , "Tolerate, put up with" , "Suffer, tolerate" , "Abide" , "Suffer or undergo" .)