Ocean movements (5)
I believe the answer is:
tides
'ocean movements' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tides that I've seen before include "Periodic rises and falls of the sea level" , "Lunar-driven ocean movements" , "Lunar-driven movements of oceans" , "Surges" , "Periodic ebbs and flows of the sea" .)