Special force releasing half the characters from custody (5)
I believe the answer is:
posse
'special force' is the definition.
(posse can mean a group of lawmen)
'releasing half the characters from custody' is the wordplay.
'releasing half the characters from' suggests halving the letters.
'custody' becomes 'possession'.
'possession' halved is 'POSSE'.
(Other definitions for posse that I've seen before include "Group formed by sheriff" , "Troop at the service of a local sheriff" , "Group aiding lawman" , "Sheriff's team" , "Sheriff's assistants" .)