Siegfried Sassoon's poem "The Hero" reflects upon the death of a soldier. In reality, the soldier was a "cold-footed, useless swine," but Brother Officer lied to the soldier's mother telling her "gallant lies" about her son's heroism.
The lines of the poem in question,
Had panicked down the trench that night the mine
Went up at Wicked Corner,
state how the solider really was.
Literally, the soldier had panicked (become overcome with fear) the night the mine blew up ("went up" is slang for exploding or catching on fire). Given the warring setting, one can assume that the mine was blown up.
A trench is a hole dug in the ground to protect soldiers during war. Wicked Corner is, most likely, the name of the area where the mine was located.
Essentially, the soldier became very scared, jumped into the long hole meant to protect him, as the mine blew up.