Cut energy, being harsh (6)
I believe the answer is:
severe
'harsh' is the definition.
(thesaurus)
'cut energy' is the wordplay.
'cut' becomes 'sever' (severing is a kind of cutting).
'energy' becomes 'e' (physics symbol as in E=mc2).
'sever'+'e'='SEVERE'
'being' acts as a link.
(Other definitions for severe that I've seen before include "Austere, spartan" , "Harsh and stringent" , "sober" , "Taxing" , "Intense; harsh" .)