Engineers, worn out, went to bed (7)
I believe the answer is:
'went to bed' is the definition.
('retire' can be a synonym of 'go to bed')
'engineers worn out' is the wordplay.
'engineers' becomes 're' (abbreviation for Royal Engineers).
'worn out' becomes 'tired' (tired can mean worn out or fatigued).
(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that I've seen before include "Turned in" , "No longer working" , "Stopped working" , "Went to bed or gave up work" , "Withdrawn from active participation" .)