Unforeseen difficulty (4)
I believe the answer is:
snag
'unforeseen difficulty' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for snag that I've seen before include "Hitch, unexpected problem" , "Disadvantage - small hole in fabric" , "Unforeseen obstacle" , "Stumbling block" , "Tear - catch" .)