Sappers, weary, go to bed (6)
I believe the answer is:
retire
'go to bed' is the definition.
(synonyms)
'sappers weary' is the wordplay.
'sappers' becomes 'RE' (nickname for the Royal Engineers).
'weary' becomes 'tire' (synonyms).
're'+'tire'='RETIRE'
(Other definitions for retire that I've seen before include "Get your head down" , "Cease working" , "Withdraw from active participation" , "Step down" , "call it a day" .)