It flows in the Seine (3)
I believe the answer is:
eau
'it flows in the seine' 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 "French water - de Cologne?" , "Perrier water" , "French water, may be Cologne" , "Quick to notice things" , "-- de vie" .)