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