Sound of falling water? Check (4)
I believe the answer is:
rein
'check' is the definition.
(both can mean to control or limit)
'sound of falling water?' is the wordplay.
'sound of' indicates a 'sounds like' (homophone) clue.
'falling water?' becomes 'rain' (I've seen this before).
'rain' is a homophone of 'REIN'.
(Other definitions for rein that I've seen before include "Free ... is carte blanche" , "Strap on a bridle" , "Check; control" , "Harness item" , "Curb" .)