Engineers, 50, relieved to be freed (8)
I believe the answer is:
released
'be freed' 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' ('ease' can be a synonym of 'relieve').
're'+'l'+'eased'='RELEASED'
'to' acts as a link.
(Other definitions for released that I've seen before include "free after imprisonment?" , "Sent from prison" , "Let out - let out again" , "Discharged" , "Liberated" .)