In retreat, sappers yielding (8)
I believe the answer is:
receding
'in retreat' is the definition.
(I've seen this before)
'sappers yielding' is the wordplay.
'sappers' becomes 're' (Royal Engineers).
'yielding' becomes 'ceding' ('cede' can be a synonym of 'yield'**).
're'+'ceding'='RECEDING'
(Other definitions for receding that I've seen before include "Going back or retreating - like hairline?" , "Becoming more distant" , "Withdrawing" , "In retreat" .)