Engineers, worn out, went to bed (7)
I believe the answer is:
retired
'went to bed' is the definition.
('retire' can be a synonym of 'go to bed')
'engineers worn' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'worn' becomes 'tired' ('tire' can be a synonym of 'wear').
're'+'tired'='RETIRED'
'out' acts as a link.
(Other definitions for retired that I've seen before include "Turned in" , "No longer working" , "Stopped working" , "Went to bed or gave up work" , "Withdrawn from active participation" .)