Dislocations decorated by both clusters of self-interstitial atoms (SIAs) and small dislocation loops, are one of the microstructure features which can play an important role in post-irradiated deformation processes. The interactions between dislocations and clusters are important and are usually treated within the framework of isotropic elasticity theory. However, it is still not clear whether or not these interactions, especially for small clusters at short distances, can be treated accurately by elasticity theory. Comparative studies by atomistic simulation and elasticity theory can clarify this. Here we present a simple example of such a study where interactions between a glissile SIA cluster and an edge dislocation are studied in bcc-Fe and fcc-Cu using both techniques. In Fe we have studied the interaction of a dislocation with Burgers vector b= 1/2<111> lying along <112> direction with a SIA cluster with the same b situated at different distances below the extra half-plane. In Cu, the dislocation and cluster had b = 1/2<110> and the dislocation line was along the <112> direction. Interactions with clusters of diameter about 1nm were simulated. Elastic calculations were made within the isotropic theory with parameters estimated from atomistic simulation. The results obtained by both techniques are discussed and some preliminary conclusions for different cases are drawn.