Sad time in prison: it flies! (8)
I believe the answer is:
bluebird
'it flies' is the definition.
The definition suggests a singular noun which matches the answer.
'sad time in prison' is the wordplay.
'sad' becomes 'blue' (similar in meaning).
'time in prison' becomes 'bird' (bird is slang for time spent in prison).
'blue'+'bird'='BLUEBIRD'
Can you help me to learn more?
(Other definitions for bluebird that I've seen before include "North American songbird (once seen over Dover?)" , "Donald Campbell's speedboat - relative of thrush" , "Fast car - or boat" , "Name of the cars and boats in which Malcolm and Donald Campbell set land and water speed records" , "Famous speedboat" .)