فَإِنَّمَا هِىَ زَجْرَةٌ وَٰحِدَةٌ ۝١٣ Why is وَٰحِدَةٌ nominative feminine singular?

Surah An-Nazi'at — Part 2

• It is the subject of an omitted verb
• It is governed by a preposition
• It is the adjective of nominative feminine زَجْرَةٌ
• It is a circumstantial accusative