Calorimetric and x-ray diffraction experiments are combined to characterize the entrance of a cyanoadamantane plastic crystal into a glassy crystal state upon cooling. As usually observed for glass-forming liquids, the configurational entropy of this glassy crystal is shown to strongly decrease on approaching the glass transition. Taking advantage of the underlying lattice of this peculiar glass, we could identify the structural origin of this entropy change. It corresponds clearly to the development of an antiferroelectric short-range order which is the precursor of a transient metastable state recently described. The analysis of the results, in the framework of the Adam and Gibbs theory, provides a new picture of the cooperative processes expected in glasses.