“go up to the forest.” The Promised Land was not a desert, but a well-watered piece of land. However, even by the Roman period, the clearing of the land was quite extensive. It was during the Roman control of Israel that the lions disappeared from Israel because so many of them were used in gladiator games.
“clear land.” The word “land” is not in the text, the Hebrew is simply “create” or “make” (#01254 bara), so the translation could well be, “make room,” or “make space,” as well as “clear land.” The idea is that the people wanted more room, so Joshua said go and make some in the land you have not taken possession of yet.
“Perizzite.” A tribe of unknown origin in that by the time of Joshua lived in the hill country of Judah and Ephraim. See commentary on Joshua 9:1.
“if the hill country of Ephraim is too narrow.” The allotment came from God, and Joshua is not going to say it is not good enough, so he uses “if” the hill country.