Prison sentence swift, perhaps? (4)
I believe the answer is:
bird
I believe this is a double definition.
'prison sentence' is the first definition.
(to 'do bird' is to serve a prison sentence)
'swift perhaps?' is the second definition.
(swift is a kind of bird)
(Other definitions for bird that I've seen before include "that flies" , "Egg-laying creature" , "Flier - spell in prison" , "porridge" , "Prison sentence - hawk, perhaps" .)