Sappers become weary, go to bed (6)
I believe the answer is:
retire
'go to bed' is the definition.
('retire' can be a synonym of 'go to bed')
'sappers become weary' is the wordplay.
'sappers' becomes 're' (Royal Engineers).
'become weary' becomes 'tire' (I've seen this before).
're'+'tire'='RETIRE'
(Other definitions for retire that I've seen before include "Give up the job and go to bed" , "Withdraw - give up work - go to bed" , "Give up working and go to bed" , "Stop doing something permanently" , "call it a day" .)