كَعَصْفٍ

كَعَصْفٍ is a phrase (شِبْهُ الجُمْلَةِ), a compound consisting of a particle of pulling down (preposition) and a genitive noun

عَصْفٌ is the natural state of this indefinite noun and it changes to كَعَصْفٍ due to the preposition that precedes it

PhraseParticle/NounNoun Particle 
شِبْهُ الجُمْلَةِجَارٌ وَمَجْرُوْرٌاسْمٌ مَجْرُوْرٌحَرْفُ جَرٍّ
PhraseCompoundGenitive NounParticle of pulling down
  
like strawكَعَصْفٍ=عَصْفٍ+كَ
like strawstrawlike
Surah/Below is the only verse in the holy Quran containing the genitive particle and genitive noun compound كَعَصْفٍ
Verse
105-5Thus, making them like devoured cropفَجَعَلَهُم كَعَصْفٍ مَّأْكُوْلٍ