Ocean motions (5)
I believe the answer is:
tides
'ocean motions' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tides that I've seen before include "drifts" , "Periodic rises and falls of the sea level" , "Critical moments" , "They ebb and flow" , "Lunar-driven ocean movements" .)