Last under hectic energy (6)
I believe the answer is:
endure
'last' is the definition.
(I know that last can be written as endure)
'under hectic energy' is the wordplay.
I cannot really understand how this works, but
an anagram of 'under' is 'endur' which is present in the answer.
'energy' could be 'e' (physics symbol as in E=mc2) and 'e' is found within the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for endure that I've seen before include "Stomach" , "Last or undergo" , "Bear without giving up" , "Last (anag.)" , "Stand" .)