Water line? (4)
I believe the answer is:
wake
'water line?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for wake that I've seen before include "Vigil; disturbed water" , "Come to - track of boat" , "Traditional watch over corpse before burial" , "Open one's eyes - funeral party" , "Ship's wash" .)