Crossword Genius

Sad time in prison that flies (8)

Ross

I believe the answer is:

bluebird

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'prison that flies' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

'sad time' is the wordplay.
'sad' becomes 'blue' (similar in meaning).
'time' becomes 'bird' (both are slang for a prison sentence).
'blue'+'bird'='BLUEBIRD'

'in' acts as a link.

Can you help me to learn more?

(Other definitions for bluebird that I've seen before include "American singer" , "Famous speedboat" , "American avian" , "symbol of happiness" , "American member of thrush family (seen over Dover?)" .)

I've seen this clue in The Sun.
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