Difficult, tough (4)
I believe the answer is:
hard
I believe this clue is a double definition.
'difficult' is the first definition.
(I know that difficult can be written as hard)
'tough' is the second definition.
(I know that tough can be written as hard)
(Other definitions for hard that I've seen before include "Novel featuring Mr Gradgrind" , "Unyielding" , "Firm; demanding; stern" , "Tough; rigid" , "Difficult; rigid" .)