"古代计时用的滴漏" can be translated into English as "the ancient water clock (clepsydra) used for timekeeping".
Here, "clepsydra" is the technical term for a water clock in English, though it's less commonly used in everyday conversation compared to "water clock." If you prefer a more straightforward and commonly understood term, "ancient water clock for timekeeping" works perfectly fine.
Both versions convey the meaning accurately, with the first being slightly more precise in terminology.