Cut off energy -- that's hard to endure (6)
I believe the answer is:
severe
'hard to endure' is the definition.
(I've seen this before)
'cut off energy' is the wordplay.
'cut off' becomes 'sever' (I've seen this before).
'energy' becomes 'e' (physics symbol as in E=mc2).
'sever'+'e'='SEVERE'
'that's' acts as a link.
(Other definitions for severe that I've seen before include "Drastic" , "Austere, harsh (6)" , "Harsh, rigorous" , "Very bad" , "Arduous - dangerous" .)