Concerned with rental agreement that’s free (7)
I believe the answer is:
release
'free' is the definition.
('release' can be a synonym of 'free')
'concerned with rental agreement' is the wordplay.
'concerned with' becomes 're'.
'rental agreement' becomes 'lease' (I've seen this before**).
're'+'lease'='RELEASE'
'that's' is the link.
(Other definitions for release that I've seen before include "Possibly film" , "put out" , "New film" , "Free - delivery" , "Authorise publication" .)