Sentence:  "Your father has told me all about it. Did you ever hear of anything so ridiculous?"/ "All about what?—all about what?" said Delia, whose attempt to represent happy ignorance seemed likely to be spoiled by an <WSD>intromission</WSD> of ferocity. She might succeed in appearing ignorant, but she could scarcely succeed in appearing happy. 
Meaning of the word marked with <WSD> tags:  putting one thing into another; insertion