Sentence:  American courts sometimes use cherry in place of apple, but the latter fruit vastly predominates. The American version is that rare <WSD>set phrase</WSD> that is not so well set, variations on the phrase being more common than the main phrase itself. 
Meaning of the word marked with <WSD> tags:  An established expression whose wording is subject to little or no variation, and which may or may not be idiomatic.