What whistled when the pretty dog went by? (9)
I believe the answer is:
wolfhound
'what whistled when the pretty' is the definition.
I can't tell whether this defines the answer.
'dog went by?' is the wordplay.
'dog' becomes 'wolf' (I have seen 'Wild dog ' mean 'wolf' so perhaps 'dog' could also mean 'wolf').
'went by?' becomes 'hound' (I can't justify this - if you can you should give a lot more credence to this answer).
'wolf'+'hound'='WOLFHOUND'
Can you help me to learn more?
(Other definitions for wolfhound that I've seen before include "Large breed of dog" , "Large dog, maybe Irish" , "Largest breed of dog" .)