不对,正确的是This is the watch which Tom is looking for.
或许你会问为什么,也看到有把介词和which连用在一起的,那是另外一种情况,因为look for 是固定词组,不能拆开.如果是 I have many friends,of whom some are businessmen.这里的介词和定语从句引导词连用,是因为of和whom产生从属关系,所以连在一起,而你所说的for和which没有任何从属关系,而是和look产生固定词组关系,所以得把for放在句末.