Wide area of plant endlessly filling river (7)
I believe the answer is:
expanse
'wide area' is the definition.
(I've seen this before)
'plant endlessly filling river' is the wordplay.
'plant endlessly' becomes 'pans' (I can't explain this - if you can you should give a lot more credence to this answer).
'filling' means one lot of letters goes inside another.
'river' becomes 'exe' (River Exe).
'pans' inserted into 'exe' is 'EXPANSE'.
'of' is the link.
Can you help me to learn more?
(Other definitions for expanse that I've seen before include "considerable area" , "Wide area of open land or space" , "Swathe" , "Stretch - extent" , "Wide continuous area" .)