Engineers get contract free (7)
I believe the answer is:
release
'free' is the definition.
(to release someone is to free them)
'engineers get contract' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'get' means one lot of letters go next to another.
'contract' becomes 'lease' (lease is a kind of contract).
're'+'lease'='RELEASE'
(Other definitions for release that I've seen before include "put out" , "press announcement" , "Authorise publication" , "Free from confinement" , "New film" .)