لِيَبْكِ يَزِيدَ ضَارِعٌ لِخُصُومَةٍ
Let the needy litigant mourn Yazīd.
The text reads «لِيَبْكِ», not «لَبَّيْكَ». The initial lām expresses a command/wish; «يَبْكِ» is a jussive imperfect of the root b-k-y, with the weak final letter dropped. «يَزِيدَ» is a proper name and, in one reading, an accusative object; «ضَارِعٌ» is the nominative agent.
The normal-order paraphrase is لِيَبْكِ ضَارِعٌ يَزِيدَ — let a needy man mourn Yazīd.
Moving the object ahead of the agent is possible in Arabic and helps bring Yazīd to attention. The grammatical issue concerns who weeps and who is mourned, and therefore cannot be taught from the incorrect word «لَبَّيْكَ».