Sink, settle (7)
I believe the answer is:
subside
I believe this clue is a double definition.
'sink' is the first definition.
(I know that subside is a more specific form of the action sink)
'settle' is the second definition.
(I know that settle can be written as subside)
(Other definitions for subside that I've seen before include "Become less intense" , "(Of floods) go down" , "decline" , "(Flood) go down" , "Fall back" .)