A theory of anomalous x-ray transmission for crystals containing defect clusters of radius much less than an extinction distance is developed in a way analogous to the treatment of the thermal vibrations. A statistical distribution of the clusters is assumed, and only the coherent wave is considered. Two cluster models are discussed. The first represents a "loose" cluster of point defects, the second a dislocation loop. The absorption consists of two parts, the photoelectric absorption and the diffuse-scattering absorption. The photoelectric absorption is essentially given by a Debye-Waller factor resulting from the static lattice displacements of the clusters. The diffuse-scattering absorption is mainly determined by the very intense diffuse scattering in the wings of the Bragg reflection. Whereas for small clusters the photoelectric absorption is most important, for very large clusters the diffuse-scattering absorption dominates. Compared to the case of isolated point defects, the absorption can increase by as much as a factor of 105, if a given number of point defects cluster and, for instance, build dislocation loops.