Water down funnel you invert, all ends in sink (6)
I believe the answer is:
dilute
'water down' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for dilute that I've seen before include "Make (a liquid) thinner" , "Reduce in strength or concentration" , "having less concentration" , "Thin down by adding water" , "not neat" .)