Hidden danger (7)
I believe the answer is:
pitfall
'hidden danger' is the definition.
(pitfalls are unexpected dangers)
This is all the clue.
(Other definitions for pitfall that I've seen before include "Trap in the form of a concealed hole" , "Hazard" , "Unsuspected danger or difficulty - for miner?" , "Unforeseen difficulty" , "Unexpected problem" .)