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What whistled when the pretty dog went by? (9)

Ross

I believe the answer is:

wolfhound

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in the Evening Standard.
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