Sad time in prison that flies (8)
I believe the answer is:
bluebird
'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?)" .)