Impervious, unyielding (9)
I believe the answer is:
I believe this clue is a double definition.
'impervious' is the first definition.
(similar in meaning)
'unyielding' is the second definition.
(Other definitions for resistant that I've seen before include "unaffected" , "Rebel" , "Withstanding adverse conditions" , "Not easily convinced" , "Impervious (to corrosion, heat, etc)" .)