Obstacle, difficulty (6)
I believe the answer is:
hurdle
This clue is a double definition.
'obstacle' is the first definition.
(I know that hurdle is a type of obstacle)
'difficulty' is the second definition.
(I know that hurdle is a type of obstruction)
(Other definitions for hurdle that I've seen before include "Barrier (to be jumped)" , "Light movable barrier in certain races" , "Problem to overcome" , "Obstacle used in races" , "Vault" .)