Engineers, 50, relieved to be set free (8)
I believe the answer is:
released
'be set free' is the definition.
(I've seen this before)
'engineers 50 relieved' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'50' becomes 'l' (Roman numeral).
'relieved' becomes 'eased' (easing is a kind of relieving).
're'+'l'+'eased'='RELEASED'
'to' acts as a link.
(Other definitions for released that I've seen before include "Freed from confinement" , "Liberated" , "Sent from prison" , "Let out - let out again" , "free after imprisonment?" .)