Decline accompanying soldiers in retreat (6)
I believe the answer is:
wither
'decline' is the definition.
(I've seen this before)
'accompanying soldiers in retreat' is the wordplay.
'accompanying' becomes 'with' (I've seen this in another clue).
'soldiers' becomes 're' (Royal Engineers).
'in retreat' is a reversal indicator.
're' backwards is 'er'.
'with'+'er'='WITHER'
(Other definitions for wither that I've seen before include "Dry up, decay" , "Droop, shrivel" , "(Of a plant) shrivel up" , "Atrophy" , "Shrivel and decay" .)