American stars swim in large, extremely picturesque, river (3,6)
I believe the answer is:
big dipper
'american stars swim' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'large extremely picturesque river' is the wordplay.
'large' becomes 'big' ('big' can be a synonym of 'large').
'extremely picturesque river' becomes 'dipper' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'big'+'dipper'='BIG DIPPER'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for big dipper that I've seen before include "Familiar US stars" , "Large bird" , "Amusement park attraction" , "Stars in America" , "Roller coaster - The Plough (in the US)" .)