"A duck", I croon, "swimming on a river" (7)
I believe the answer is:
orinoco
'river' is the definition.
(Orinoco River is an example)
'a duck i croon swimming' is the wordplay.
'a duck' becomes 'o'.
'swimming' indicates an anagram.
'i'+'croon'='icroon'
'icroon' with letters rearranged gives 'rinoco'.
'o'+'rinoco'='ORINOCO'
'on a' is the link.
(Other definitions for orinoco that I've seen before include "may be seen running through South America" , "Common character" , "Great river flowing through Venezuela to Atlantic" , "River, rising in Venezuela, forming part of its border with Colombia and flowing into the Atlantic" , "O, I croon about the river" .)