Sappers become weary and retreat (6)
I believe the answer is:
retire
'retreat' is the definition.
(thesaurus)
'sappers become weary' is the wordplay.
'sappers' becomes 're' (Royal Engineers).
'become weary' becomes 'tire' (I've seen this before).
're'+'tire'='RETIRE'
'and' is the link.
(Other definitions for retire that I've seen before include "Leave job permanently (6)" , "Leave work; go back" , "Stop climbing the ladder" , "go upstairs?" , "Give up working and go to bed" .)