Liberate at no cost (4)
I believe the answer is:
free
I believe this clue is a double definition.
'liberate' is the first definition.
(I know that liberate is a more specific form of 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 "Allowed to do as one pleases" , "Not employed" , "Deliver" , "As cheap as can be!" , "Gratis, for nothing" .)