Learn English – What does it mean “To hold up against scrutiny”

In this context, the word above means better than. If someone is above doing something, they feel as though the activity in question is not up to their standards. It is also said that the activity a person is above is beneath them.

This is supported by the fourteenth definition of above, which states:

14) of too fine a character for: He is above such trickery.

In your question, the person negated the phrase. By saying she is not above breaking a rule occasionally, she means that she does not consider herself too good or proper to break a grammatical rule when it enhances her clarity.

The phrase in the book is:

So they think you're dead, Andrews, and they have now played their last card. So we must hold ours back. You're going to remain dead—for a little while longer.

They're using an analogy to a card game such as poker. They have put their last card down, or made their intentions clear.

But they're going to keep their last card a little longer, and keep their secret from the others: that Andrews is not really dead.