Notice difficulty (4)
I believe the answer is:
spot
This clue is a double definition.
'notice' is the first definition.
(I've seen this before)
'difficulty' is the second definition.
(as in a spot of difficulty)
(Other definitions for spot that I've seen before include "Difficult situation" , "See - blemish" , "Small mark; see" , "station" , "small amount" .)