Rare vacant tenancy agreement — it's free! (7)
I believe the answer is:
'free' is the definition.
('release' can be a synonym of 'free')
'rare vacant tenancy agreement' is the wordplay.
'vacant' means to remove the middle letters.
'tenancy agreement' becomes 'lease' (I've seen this before).
'rare' with its centre taken out is 're'.
'it's' is the link.
(Other definitions for release that I've seen before include "put out" , "press announcement" , "Set free - publish" , "Liberate" , "Let go" .)