Release at no cost (4)
I believe the answer is:
free
I believe this clue is a double definition.
'release' is the first definition.
(I know that release can be written as free)
'no cost' is the second definition.
(I've seen this in other clues)
'at' is the link.
(Other definitions for free that I've seen before include "Not employed" , "See 8" , "Unconfined" , "unlike battery hens" , "Reef (anag.)" .)