Thwarts the new mistress who loses her head (7)
I believe the answer is:
resists
'thwarts' is the definition.
The answer and definition can be both to do with social activities as well as being verbs in their -s form.
Maybe you can see an association between them that I can't see?
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for resists that I've seen before include "Tries to stop" , "Defends" , "Sisters (anag.)" , "Is reluctant" , "Is hostile to" .)