Confine water source, restricting river (4)
I believe the answer is:
trap
'confine' is the definition.
(trapping is a kind of confining)
'water source restricting river' is the wordplay.
'water source' becomes 'tap'.
'restricting' indicates putting letters inside.
'river' becomes 'r'.
'tap' enclosing 'r' is 'TRAP'.
(Other definitions for trap that I've seen before include "Device to catch prey" , "Driver's obstacle" , "Pitfall" , "that may catch a speeding motorist" , "ladder" .)