Ebb and flow of the sea (4)
I believe the answer is:
tide
'ebb and flow of the sea' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for tide that I've seen before include "Something going in and out" , "Current, ebb" , "Periodic rise and fall of the sea" , "What might carry" , "Rising and falling of the sea" .)