In the domain of heterogeneous model composition, semantic adaptation is the "glue" that is necessary to assemble heterogeneous models so that the resulting composed model has well-defined semantics. In this paper, we present an execution model for a semantic adaptation interface between heterogeneous models. We introduce a Domain-Specific Language (DSL) for specifying such an interface explicitly using rules, and a transformation toward the ModHel'X framework. The DSL enables the modeller to easily customise interfaces that fit the heterogeneous model at hand in a modular way, as involved models are left untouched. We illustrate the use of our DSL on a power window case study and demonstrate the importance of defining semantic adaptation explicitly by comparing with the results obtained with Ptolemy II.