Extremely low flower has greenfly? (7)
I believe the answer is:
deepest
'extremely low flower has greenfly?' is the definition.
I can't judge whether this definition defines the answer.
'flower has greenfly?' is the wordplay.
'flower' becomes 'dee' (River Dee).
'has greenfly?' becomes 'pest' (I am not sure about this - if you are sure you should believe this answer much more).
'dee'+'pest'='DEEPEST'
Can you help me to learn more?
(Other definitions for deepest that I've seen before include "hard to get, in the extreme" , "Most intense" , "Most learned" , "Most low" , "most insidious" .)