Ocean currents (5)
I believe the answer is:
tides
'ocean currents' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tides that I've seen before include "They ebb and flow" , "Moon-driven ocean movements" , "Periodic ebbs and flows of the sea" , "Surges" , "Rises and falls, nautically" .)