Engineers rental agreement that’s free (7)
I believe the answer is:
release
'that's free' is the definition.
(to release someone is to free them)
'engineers rental agreement' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'rental agreement' becomes 'lease' (eg rental agreement for a flat).
're'+'lease'='RELEASE'
(Other definitions for release that I've seen before include "record" , "New film" , "Possibly film" , "Free - delivery" , "Set free - publish" .)