事從緩來 Let the Matter Come Slowly
正都在悚懼之際，又一個小妖上前道：「大王莫惱，莫怕。常言道：『事從緩來。』若是要吃唐僧，等我定個計策拿他。」正都在悚惧之际，又一个小妖上前道：「大王莫恼，莫怕。常言道：『事从缓来。』若是要吃唐僧，等我定个计策拿他。」
逐句解 Line by line
Just as they were all standing there in dread, another little imp stepped forward. "Great King, do not fret and do not be afraid.
悚懼 pairs two words for fear and describes the whole cave — the commander's alarm has spread to the staff. And a second imp comes forward, which is how this scene is built: the first one broke the plan, and this one has a better.
As the saying goes: let a matter come on slowly.
常言道 — as the common saying has it — is the standard way this novel introduces a proverb, and the nested quotation marks are the printer's, not an editor's. 事從緩來 counsels patience in a difficulty, which is exactly the advice a frightened commander needs and exactly the opening a clever subordinate wants.
If you still mean to eat the Tang Monk, let me settle on a plan for taking him."
定個計策 is to fix on a stratagem. The imp has revived the whole project in fourteen characters, and he has done it by conceding everything the last imp said and changing only the method.
The cave has a second clever imp, and the novel gives him no name either. Between them, in the space of two conversations, they take the operation away from their commander and hand it back to him improved.
The proverb is doing real work. Let the matter come slowly is not a refusal; it is a reframing. The problem was never that the monk is unattainable, only that grabbing at him is.