Mouth of river feeding water supply (4)
I believe the answer is:
trap
'mouth' is the definition.
(informal term for mouth)
'river feeding water supply' is the wordplay.
'river' becomes 'r'.
'feeding' is an insertion indicator.
'water supply' becomes 'tap' (I've seen this before).
'r' going within 'tap' is 'TRAP'.
'of' is the link.
(Other definitions for trap that I've seen before include "Rock; carriage; gin" , "that can let an actor down" , "The Web" , "Trick" , "Bunker; carriage" .)