Engineers let loose (7)
I believe the answer is:
release
'loose' is the definition.
(I've seen this before)
'engineers let' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'let' becomes 'lease' ('lease' can be a synonym of 'let').
're'+'lease'='RELEASE'
(Other definitions for release that I've seen before include "Set free - publish" , "New film" , "Free from confinement" , "Liberate" , "Free - delivery" .)