Stubborn person cracks in response to charm (6)
I believe the answer is:
amulet
'to charm' is the definition.
(amulet is a kind of charm)
'stubborn person cracks in response' is the wordplay.
'stubborn person' becomes 'mule' (I've seen this in another clue).
'cracks' means one lot of letters goes inside another (crack their way in).
'in response' becomes 'at' (I can't justify this - if you can you should believe this answer much more).
'mule' inserted inside 'at' is 'AMULET'.
Can you help me to learn more?
(Other definitions for amulet that I've seen before include "Piece of jewelry thought to be a protection against evil" , "lucky defender" , "Charm, lucky token" , "Magical charm" , "Thing worn as charm against evil" .)