La mer filler (3)
I believe the answer is:
eau
'la mer filler' 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 "French water from chateaux" , "French water, may be Cologne" , "-- de vie" , "Water in France" , "French water - de Cologne?" .)