Ebb, retreat (6)
I believe the answer is:
recede
I believe this clue is a double definition.
'ebb' is the first definition.
(I know that ebb is a more specific form of the action recede)
'retreat' is the second definition.
(I know that retreat can be written as recede)
(Other definitions for recede that I've seen before include "Start balding" , "Go backwards" , "Become more distant" , "Decline" , "Move backward" .)