Ebbing and flowing (5)
I believe the answer is:
tidal
'ebbing and flowing' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tidal that I've seen before include "Subject to ebb and flow" , "Concerning the rise and fall of the sea" , "Concerning lunar-driven ocean movements" , "To do with movement of seas" , "Subject to moon's pull" .)