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