Army group is about to take position fighting against enemy? (10)
I believe the answer is:
resistance
'fighting against enemy?' is the definition.
(resisting an enemy)
'army group is about to take position' is the wordplay.
'army group' becomes 'RE' (Royal Engineers).
'about' shows that the letters should be reversed in order.
'to take' is a charade indicator (letters next to each other).
'position' becomes 'stance' (stance is a kind of position).
'is' reversed gives 'si'.
're'+'si'+'stance'='RESISTANCE'
(Other definitions for resistance that I've seen before include "Organised covert opposition to an occupying power" , "defiance" , "Withstanding" , "immunity" , "Rebellious group" .)