top of the batting order; heart of the lineup
Refers to the first few batters in a baseball lineup, typically the strongest hitters.
If the top of the order gets hits in succession, we can expect a lot of runs.
He is a power hitter entrusted with a spot at the top of the order.
クリーンナップ specifically refers to the 3rd, 4th, and 5th batters, while 上位打線 is a broader term for the top part of the order, often including the 1st and 2nd batters as well.
Compound of 上位 (high rank, top) and 打線 (batting lineup).