Cut off energy? That's hard (6)
I believe the answer is:
severe
'hard' is the definition.
(I know that hard can be written as severe)
'cut off energy?' is the wordplay.
'cut off' becomes 'sever' (to sever is to cut).
'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 "Intense; harsh" , "Unsparing and uncompromising" , "Arduous - dangerous" , "Serious or strict" , "Harsh and stern" .)