Getting the drop on someone means that you got an advantage over them, this advantage usually involves taking action that put you ahead of your adversary in planning, timing, intelligence, among others. This term is used often in Hip Hop music by rappers. In Nas' "The Message" song, he raps " I had the drop so how the f*ck I get clapped
." Nas is literally saying that he had control over the situation, how is it that he got shot (clapped) by the enemy. This term is similar to "have (get) the jump on."