Only then did she realize that someone had it in for her.

had 是动词 have 的过去式，表示"拥有"或"怀有"。 it 在这里作为代词，指代"恶意"或"敌意"。 in 是介词，表示"针对"或"对...有"。 for 是介词，表示"对..."或"针对..."。 have it in for someone 是一个固定短语，意思是"对某人怀有恶意"或"蓄意针对某人"。