Sea's rise and fall (4)
I believe the answer is:
tide
'sea's rise and fall' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tide that I've seen before include "Ocean phase" , "Movement of seawater" , "Periodic water movement" , "Powerful trend of events" , "Time and it wait for no man" .)