Army section leads in attack, not last usually in retreat (4)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

this was my LOI, twigged eventually when I had the final A from 13d. As no doubt the setter intended, I was stuck on sections of a real army, before remembering the “Uxbridge English Dictionary” to think “army” = “of an arm”. You have the leading letters, reversed, as highlighted above.