Source of water around river mouth (4)
I believe the answer is:
trap
'mouth' is the definition.
(informal term for mouth)
'source of water around river' is the wordplay.
'source of water' becomes 'tap' (I've seen this in another clue).
'around' means one lot of letters goes inside another.
'river' becomes 'r' (abbreviation).
'tap' enclosing 'r' is 'TRAP'.
(Other definitions for trap that I've seen before include "Snare; light carriage" , "The Web" , "Lock" , "Bunker; carriage" , "ladder" .)