Extreme and difficult to endure (6)
I believe the answer is:
severe
'extreme and difficult to endure' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for severe that I've seen before include "Extreme - strict" , "Harsh and stringent" , "Austere or strict" , "Very plain in style or appearance" , "Harsh - extreme" .)