Depression; gap (4)
I believe the answer is:
hole
I believe this clue is a double definition.
'depression' is the first definition.
(depression can mean a hole or crater)
'gap' is the second definition.
(I know that hole is a type of gap)
(Other definitions for hole that I've seen before include "Aperture; flaw in a plan" , "Opening on golf course" , "Well" , "mess" , "excavation" .)