Terrain; touch down (4)
I believe the answer is:
land
I believe this clue is a double definition.
'terrain' is the first definition.
(I've seen this before)
'touch down' is the second definition.
(eg touch down in a plane)
(Other definitions for land that I've seen before include "See 22" , "Light" , "See 3" , "nation" , "Bring or come ashore" .)