Time in prison, which may fly by (4)
I believe the answer is:
bird
This clue is a double definition.
'time in prison' is the first definition.
(bird is slang for time spent in prison)
'which may fly by' is the second definition.
(birds fly)
(Other definitions for bird that I've seen before include "Prison sentence - hawk, perhaps" , "Prison time (slang)" , "A prison sentence (slang)" , "Feathered animal" , "Charlie "___" Parker - time inside" .)