Water in la mer (3)
I believe the answer is:
eau
'water in la mer' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for eau that I've seen before include "Quick to notice things" , "Popular toilet water - . . . de Cologne" , "French water from chateaux" , "French water, may be Cologne" , "Perrier water" .)