Jail unit (4)
I believe the answer is:
cell
'jail unit' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for cell that I've seen before include "Lock-up room" , "Monk's small room" , "Small room for confining a prisoner" , "Compartment in honeycomb" , "may be wet or dry?" .)