Engineers, weary, 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 weary' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'weary' becomes 'tired' (similar in meaning).
're'+'tired'='RETIRED'
(Other definitions for retired that I've seen before include "Turned in" , "Finished work" , "Pensioned off" , "Stopped work - went to bed" , "Ceased working" .)