What is an example of an idiom in Chapter 7?

An idiom is a phrase whose meaning cannot be guessed from the individual meanings of its component parts. For example, kick the bucket is an English idiom for dying—if you did not know what the phrase as a whole meant, you would never be able to guess by the denotations of each of the words involved, because literally kicking a bucket has nothing in common with passing away. In Chapter 7 of The Magician’s Nephew, while Digory is waiting for a sign of the return of the witch after she has left the house to (potentially) run amok in London, “he went into the dining-room and ‘glued his face,’ as they say, to the window.”  Here, glued his face to the window is an idiom—he of course did not literally glue his face to the glass, but rather he watched for anything out the window with unyielding vigor. So, the meaning of the phrase is not equal to the meaning of its individual parts.