"Except as a mercy from your Lord" (17:87). It contains an ellipsis; it is saying: Rather, We have revealed it to you as a mercy from your Lord—the rest of the verse.
﴿إِلَّا رَحْمَة من رَبك﴾ فِيهَا إِضْمَارٌ يَقُولُ: وَإِنَّمَا أَنْزَلْنَاهُ عَلَيْكَ رَحْمَةٍ مِنْ رَبِّكَ، الْآيَةُ.
