Prisoner to fall behind (3)
I believe the answer is:
lag
This clue is a double definition.
'prisoner' is the first definition.
(slang term)
'to fall behind' is the second definition.
('lag' can be a synonym of 'fall behind')
(Other definitions for lag that I've seen before include "Wrap to protect from frost" , "Protect from cold" , "Dawdle" , "Fall behind with gal" , "Delay" .)