38 The Israelites had arranged with the ambush that they should send up a great cloud of smoke from the city,

Read Judges 20:38 Using Other Translations

Now there was an appointed sign between the men of Israel and the liers in wait, that they should make a great flame with smoke rise up out of the city.
Now the appointed signal between the men of Israel and the men in the main ambush was that when they made a great cloud of smoke rise up out of the city
They had arranged to send up a large cloud of smoke from the town as a signal.