Flier in prison? (4)
I believe the answer is:
bird
I believe this clue is a double definition.
'flier' is the first definition.
('bird' can be a synonym of 'birdanimal')
'prison?' is the second definition.
(I've seen this before)
'in' acts as a link.
(Other definitions for bird that I've seen before include "Creature - time in prison" , "Feathered flyer" , "Feathered animal" , "Avian" , "porridge" .)