River blocker (3)
I believe the answer is:
dam
'river blocker' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for dam that I've seen before include "A mad animal mother" , "Wall restraining water" , "Barrier to contain the flow of water" , "Mother of quadruped" , "Mad as an animal mum" .)