﴿ فَعَسَىٰ رَبِّىٓ أَن يُؤْتِيَنِ خَيْرًۭا مِّن جَنَّتِكَ وَيُرْسِلَ عَلَيْهَا حُسْبَانًۭا مِّنَ ٱلسَّمَآءِ فَتُصْبِحَ صَعِيدًۭا زَلَقًا ﴾

“yet it may well be that my Sustainer will give me something better than thy garden -just as He may let loose a calamity out of heaven upon this [thy garden], so that it becomes a heap of barren dust”

maybe my Lord will give me something better than your garden this is the response to the conditional clause beginning with lawlā ‘and if only’ and unleash upon it bolts husbān is the plural of husbāna that is to say thunderbolts from the heaven so that it becomes a bare plain a piece of smooth ground upon which no foot can stand firm;