Hard to endure energy cut at first (6)
I believe the answer is:
severe
'hard to endure' is the definition.
(I've seen this before)
'energy cut at first' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'cut' becomes 'sever' (to sever is to cut or split).
'at first' says to put letters next to each other.
'e' after 'sever' is 'SEVERE'.
(Other definitions for severe that I've seen before include "Austere, serious" , "Forbidding in action or appearance" , "Drastic" , "Austere, harsh (6)" , "Harsh and punitive" .)