Difficult set (4)
I believe the answer is:
hard
This clue is a double definition.
'difficult' is the first definition.
('hard' can be a synonym of 'difficult')
'set' is the second definition.
('hard' can be similar in meaning to 'set')
(Other definitions for hard that I've seen before include "Firm, rigid, road leading to foreshore" , "impenetrable" , "Firm; demanding; stern" , "Intensely" , "Strenuous" .)