Being cut to the quick may be thus resented (6)
I believe the answer is:
keenly
'be thus resented' is the definition.
I can't tell whether this defines the answer.
'being cut to the quick' is the wordplay.
I cannot quite see how this works, but
'quick' could be 'keen' (I've seen this before) and 'keen' is located in the answer.
This explanation may well be incorrect...
'may' acts as a link.
Can you help me to learn more?
(Other definitions for keenly that I've seen before include "Eagerly, enthusiastically" , "With enthusiasm" , "With eagerness" , "In an eager manner" , "Eagerly, sharply" .)