On which there's not a score of waterfowl? (4,4)
I believe the answer is:
duck pond
'on which there's not a' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'score of waterfowl?' is the wordplay.
'score' becomes 'duck' (duck is a kind of score).
'of waterfowl?' becomes 'pond' (I can't justify this - if you can you should give a lot more credence to this answer).
'duck'+'pond'='DUCK-POND'
Can you help me to learn more?
(Other definitions for duck pond that I've seen before include "swimming facility" , "where Drake could be seen" , "Piece of water, maybe on village green" .)