Intractable (9)
'intractable' is the definition.
(associated in meaning)
This is the entire clue.
(Other definitions for obstinate that I've seen before include "Firm" , "Like mules" , "Stubbornly persistent" , "Inflexible" , "Cussed" .)
(Other definitions for difficult that I've seen before include "Requiring much effort" , "Hard, not easy" , "Demanding" , "Challenging" , "Trying" .)