Go out, as the tide (3)
I believe the answer is:
ebb
'go out as the tide' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ebb that I've seen before include "Go out or recede like tide" , "(Of tidewater) recede" , "Withdraw heartless" , "Drain away; decline" , "state of decline" .)