Self-willed (10)
I believe the answer is:
headstrong
'self-willed' is the definition.
(I know that self-willed can be written as headstrong)
This is all the clue.
(Other definitions for headstrong that I've seen before include "Showing wilfulness" , "Wilfully determined" , "Wilful, determined" , "obstinately self-willed" , "pig-headed" .)