Let loose by heads of unusually nice prison in Norway (5)
I believe the answer is:
'let loose' is the definition.
(to remove or loosen a pin)
'heads of unusually nice prison in norway' is the wordplay.
'heads of' suggests taking the first letters.
'norway' becomes 'n'.
The first letters of 'unusually nice prison in' is 'unpi'.
'by' is the link.
(Other definitions for unpin that I've seen before include "Things tied together" , "release" , "Loosen" , "free" , "Remove fastening from" .)