Lait, mostly (3)
I believe the answer is:
eau
'lait mostly' 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" , "Water in France" , "Perrier water" , "French water, may be Cologne" , "Popular toilet water - . . . de Cologne" .)