Crossword Genius

Engineers, worn out, went to bed (7)

Ross

I believe the answer is:

retired

'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).
're'+'tired'='RETIRED'

(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that I've seen before include "Turned in" , "No longer working" , "Stopped working" , "Went to bed or gave up work" , "Withdrawn from active participation" .)

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