فِي عَمَدٍ

فِي عَمَدٍ is a phrase (شِبْهُ الجُمْلَةِ) consisting of a particle of pulling down (preposition) and a genitive noun

PhraseParticle / NounNoun Particle
شِبْهُ الجُمْلَةِجَارٌ وَمَجْرُوْرٌاسْمٌ مَجْرُوْرٌحَرْفُ جَرٍّ
PhraseCompoundGenitive NounParticle of pulling down
  
in columnsفِي عَمَدٍ=عَمَدٍ+فِي
in columnscolumnsin
Surah/Below is the only verse in the holy Quran containing the preposition and genitive noun compoundفِي عَمَدٍ
Verse
104-9in extended columnsفِي عَمَدٍ مُّمَدَّدَةٍ