Release men thus possessing a legal right (8)
I believe the answer is:
easement
'a legal right' is the definition.
(right to use someone else's land)
'release men thus possessing' is the wordplay.
'possessing' indicates a hidden word (I've seen 'possess' mean this).
'EASEMENT' can be found hidden inside 'release men thus'.
(Other definitions for easement that I've seen before include "right of passage" , "The legal right to cross or use another's land" , "landowner's right" , "giving right of access to adjacent area?" , "Right to cross another's property; peace" .)