Leafstalk trailing river duck (7)
I believe the answer is:
pochard
'duck' is the definition.
(pochard is a kind of duck)
'leafstalk trailing river' is the wordplay.
'leafstalk' becomes 'chard'.
'trailing' says to put letters next to each other.
'river' becomes 'po' (Italian river).
'chard' put after 'po' is 'POCHARD'.
(Other definitions for pochard that I've seen before include "bird" , "Type of diving duck" , "diver" , "Spanish wine" .)