Allow to escape (7)
(Other definitions for let slip that I've seen before include "Fail to keep Secret" , "Reveal accidentally, or allow to skid?" , "Allow to escape (information etc)" , "Allow to fall or reveal a secret accidentally" , "accidentally revealed" .)
(Other definitions for release that I've seen before include "Let go" , "Prisoner eagerly awaits this" , "put out" , "Issue" , "Set free - publish" .)