Ocean's motions (5)
I believe the answer is:
'ocean's motions' 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 "Lunar-driven sea movements (5)" , "Critical moments" , "Edits (anag.)" , "Lunar-driven movements of oceans" , "Sea movements caused by moon" .)