Soldiers, exhausted, 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')
'soldiers exhausted' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'exhausted' becomes 'tired' ('tire' can be a synonym of 'exhaust').
're'+'tired'='RETIRED'
(Other definitions for retired that I've seen before include "Finished work" , "went back" , "No longer working" , "Gone to bed" , "Withdrawn from active participation" .)