Person search often requires a query photo of the target person. However, in many practical scenarios, there is no guarantee that such a photo is always available. In this paper, we define the problem of sketch based person search, which uses a sketch instead of a photo as the probe for retrieving. We tackle this problem in a weak supervision setting and propose a clustering and feature attention based weakly supervised learning framework, which contains two stages of pedestrian detection and sketch based person re-identification. Specially, we introduce multiple detectors, followed by fuzzy c-means clustering to achieve weakly supervised pedestrian detection. Moreover, we design an attention module to learn discriminative features in subsequent re-identification network. Extensive experiments show the superiority of our method.