The first systematic study of the serendipitous X-ray source density around 10 high z (0.24<z<1.2) clusters performed with Chandra is presented. A factor \~2 overdensity has been found in 4 cluster fields. The result is statistically highly significant (at >2sigma per field, and >5sigma overall) only on scale of 8'X8' and peculiar to cluster fields. The blank field fluctuations, i.e. the most sensitive measurements to date of the X-ray cosmic variance on this scale, indicate (1 sigma) variations within ~15-25% around the average value. The first marginal evidence that these overdensities increase with cluster redshift is also presented. We speculate that the most likely explanation is that the sources in excess are AGNs tracing filaments connected to the clusters. If the association of the overdensities with large-scale structures and their positive relation with cluster redshift is confirmed, these studies could represent a new, direct, way of mapping the cosmic web and its cosmological parameters at high z.