Obscure bird seen over dock -- pie in the sky? (5-6-4)

I believe the answer is:

cloud-cuckoo-land

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'pie in the sky?' is the definition.
(both can mean crazy or unrealistic)

'obscure bird seen over dock' is the wordplay.
I cannot quite see how this works, but
'obscure' could be 'cloud' (clouding is a kind of obscuring) and 'cloud' is found within the answer.
'bird' could be 'cuckoo' (cuckoo is a kind of bird) and 'cuckoo' is located in the answer.
'dock' could be 'land' (both can mean to arrive on dry land) and 'land' is located in the answer.
No letters remain.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Telegraph.

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