Caller has prisoner finally freed from nick (5)
I believe the answer is:
bugle
'caller' is the definition.
(a bugle is used for military calls)
I don't understand the remainder of the clue.
(Other definitions for bugle that I've seen before include "ornament for dress" , "Bit of brass" , "costume ornament?" , "Instrument used for military signals" , "Mil. trumpet" .)