Perish alongside retreating army corps (6)
I believe the answer is:
wither
'perish' is the definition.
(I've seen this before)
'alongside retreating army corps' is the wordplay.
'alongside' becomes 'with' (I've seen this before).
'retreating' is a reversal indicator.
'army corps' becomes 'RE' (Royal Engineers).
're' in reverse letter order is 'er'.
'with'+'er'='WITHER'
(Other definitions for wither that I've seen before include "Dry up and die" , "Writhe (anag.)" , "Waste" , "Shrink, shrivel up" , "Dry up, shrivel (6)" .)