Drop or sink (7)
I believe the answer is:
I believe this is a double definition.
'drop' is the first definition.
(I know that drop is a more specific form of the action descend)
'sink' is the second definition.
(I know that sink is a more specific form of the action descend)
(Other definitions for descend that I've seen before include "Move downwards" , "Plunge" , "Come down" , "Climb down" , "Move downward" .)