Here "gate" and "city" represent the people at the city gates and in the cities. "Howl, you people at the city gates; cry, you people in the cities" (See: [[rc://en/ta/man/translate/figs-metonymy]])
Melting away represents becoming weak because of fear. AT: "you will grow weak with fear" (See: [[rc://en/ta/man/translate/figs-metaphor]])
# For out of the north comes a cloud of smoke
This implies that a large army is coming from the north. AT: "For from the north comes a large army with a cloud of smoke" (See: [[rc://en/ta/man/translate/figs-explicit]])
# a cloud of smoke
Possible meanings are 1) this represents a cloud of dust that the army stirs up as they travel on dry dirt roads. AT: "a cloud of dust" or 2) there is a lot of smoke because of all the things that the army destroys and burns. AT: "much smoke" (See: [[rc://en/ta/man/translate/figs-metaphor]])
# there is no straggler in his ranks
"no one in his ranks walks slowly behind the others"
# How will they respond to the messengers of that nation?
The author uses this question to introduce his instruction about how the Israelites should speak to the messengers. AT: "This is how we will answer the messengers of Philistia." (See: [[rc://en/ta/man/translate/figs-rquestion]])