As Irene thrashes around in her feelings of jealously and betrayal around Clare, Larsen uses an idiom to express her frustration with Clare's excessive, limitless desires:
The trouble with Clare was, not only that she wanted to have her cake and eat it too, but that she wanted to nibble at the cakes of other folk as well.
The idiom "have your cake and eat it too" is still in common usage. It refers to wanting to enjoy multiple benefits that are usually mutually exclusive: usually a person would have to pick one. Irene feels that Clare is not satisfied with just passing as white; she also seeks an acceptance within the Black community that passing would usually prevent. Irene thinks that Clare wants to exist in both worlds without giving up either of them, which she feels is unfair. Additionally, the extension of the idiom—Clare wanting to "nibble at the cakes of other folk as well"—points to Irene’s certainty that Clare is having an affair with Brian. He is Irene’s "cake" in this passage, and she feels that Clare is trying to get access to him she shouldn’t have. All of this only amplifies Irene's perception of Clare as someone who routinely takes more than her fair share.