Headstrong (6)
(Other definitions for obdurate that I've seen before include "Implacable" , "uncommonly tough" , "Stubborn, unpersuadable" , "Stubbornly resistant" , "Impervious to persuasion" .)
'headstrong' is the definition.
(I know that headstrong can be written as wilful)
This is all the clue.
(Other definitions for wilful that I've seen before include "Deliberate - unreasonably stubborn" , "Disposed to disobedience and opposition" , "Done deliberately, not an accident" , "Perverse" , "Deliberately disobedient" .)