Soldiers' rent is free (7)
I believe the answer is:
release
'is free' is the definition.
(to release someone is to free them)
'soldiers' rent' is the wordplay.
'soldiers'' becomes 'RE' (Royal Engineers).
'rent' becomes 'lease' (I've seen this before).
're'+'lease'='RELEASE'
(Other definitions for release that I've seen before include "Authorise publication" , "Prisoner eagerly awaits this" , "Free from confinement" , "Set free - publish" , "Liberate" .)