Duck eating river weed (4)
I believe the answer is:
drip
'weed' is the definition.
(thesaurus)
'duck eating river' is the wordplay.
'duck' becomes 'dip' (both can mean to plunge into water).
'eating' indicates putting letters inside.
'river' becomes 'r' (abbreviation).
'dip' enclosing 'r' is 'DRIP'.
(Other definitions for drip that I've seen before include "Means of intravenous feeding" , "Weak and ineffectual person" , "Soppy person" , "Liquid falling in small golbules, or a dull person" , "Wet - sign of faulty washer" .)