A disastrous blunder, telling these men in 7 21! (5,7)
I believe the answer is:
light brigade
If this is the answer, I'm afraid I cannot explain it.
(Other definitions for light brigade that I've seen before include "Ill-fated cavalry unit in Crimean War" , "The charge of the -- -- (Tennyson)" , "The famous six hundred" , "The "noble six hundred"" , "Unit commemorated by 1" .)