Motion of the ocean (4)
I believe the answer is:
tide
'motion of the ocean' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tide that I've seen before include "Regular movement" , "''There is a ... in the affairs of men'' (Julius Caesar)" , "Sea's ebb and flow" , "What might carry" , "Ocean movement" .)