Army prisoner restraint is missing lower end (6)
I believe the answer is:
legion
'army' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
(Other definitions for legion that I've seen before include "Vast multitude" , "France has a foreign one" , "Part of an army; many" , "Large unit of the old Roman army" , "Roman military body of men" .)