Difficult to endure cut close to bone (6)
I believe the answer is:
severe
'difficult' is the definition.
(I've seen this before)
'endure cut close to bone' is the wordplay.
I cannot really understand how this works, but
'cut' could be 'sever' (severing is a kind of cutting) and 'sever' is found in the answer.
A single letter 'e' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
'to' is the link.
Can you help me to learn more?
(Other definitions for severe that I've seen before include "Harsh and exacting" , "Critical" , "Extreme and difficult to endure" , "Austere, serious" , "Harsh - extreme" .)