Backward flow (3)
I believe the answer is:
ebb
'backward flow' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ebb that I've seen before include "Not to wax" , "Withdraw heartless" , "Initially Mrs Browning's" , "Type of tide" , "Receding movement, as of a tide" .)