بِالمِرْصَادِ

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

PhraseParticle / NounNoun Particle
شِبْهُ الجُمْلَةِجَارٌ وَمَجْرُوْرٌاسْمٌ مَجْرُوْرٌحَرْفُ جَرٍّ
PhraseCompoundGenitive NounParticle of pulling down/Preposition
  
is ever watchfulبِالمِرْصَادِ=المِرْصَادِ+بِ
is ever watchfulever watchfulis
Surah/Below is the only verse in the holy Quran containing the phraseبِالمِرْصَادِ
Verse
89-14Indeed, your lord is surely Ever Watchfulإِنَّ رَبَّكَ لَبِالمِرْصَادِ