燒作火池坑 Burnt Into A Fire Pit
行者叫：「八戒，不必築樹，但得此物填塞洞裡，放起火來，燒得個乾淨。」火一起，果然把一座清華妖怪宅，燒作火池坑。行者叫：「八戒，不必筑树，但得此物填塞洞里，放起火来，烧得个干净。」火一起，果然把一座清华妖怪宅，烧作火池坑。
逐句解 Line by line
Pilgrim called, "Bajie, there is no need to dig at the trees. Just get this stuff stuffed into the cave, and start the fire, and it will burn the place clean."
燒得個乾淨 takes de, the connector that hangs a result on a verb: burn it clean. 乾淨 is gān jìng. 不必築樹: Bajie has been at the willows for two passages and is told to stop. The trees were never the problem.
The fire went up, and sure enough it turned that whole Clear Splendour fiend-house into a burning pit.
火池坑, a fire-pool-pit, is three words for a hole full of fire piled on top of each other, and the effect is of something hastily and thoroughly ruined. The name 清華, Clear Splendour, appears one last time, attached now to 妖怪宅, a fiend-house. The chapter has called this place a manor, a cave, an immortal residence, and now this.
The garden the poem spent six lines praising — the jade grasses, the rare flowers, the paradise that was no worse than Penglai — is filled with year-old brushwood and set alight. The novel does not mention any of that while it burns.
Wukong's phrase for the operation was sweeping the place clean, and the word he used, 乾淨, is the ordinary domestic one — what you do to a floor. The whole cave is a mess to be tidied.