They lower the flow of water (5)
I believe the answer is:
'water' is the definition.
(falls is a kind of water)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for falls that I've seen before include "suffers defeat" , "See 6 Down" , "Victoria or Niagara, for example" , "Goes down" , "Cascade" .)