نَفْخَةٌ وَاحِدَةٌ — One single blast.
نَفْخَةٌ: feminine singular indefinite nominative noun.
وَاحِدَةٌ: feminine singular indefinite nominative adjective.
In the Qurʾānic clause from which the book quotes this phrase, nafkhatun is nāʾib fāʿil, and wāḥidatun is its adjective of emphasis.