Modern synchrotron total scattering measurements can give key insights into molecular clustering in liquids. Here we combine modern synchrotron total scattering, small angle x-ray scattering, molecular dynamics, and liquid-liquid equilibrium techniques to look for direct evidence of such dynamic specific molecular clustering in a representative LCST system, mixtures of diisopropylamine (DIPA) and water, across a range of temperatures and mixture compositions. We uncover convincing evidence for the existence of extended, well-defined molecular cluster structures (MCSs) involving DIPA and water. No such well-defined molecular clustering is present in pure DIPA. Thus, the water acts to order the DIPA molecules in the mixture, which will reduce their entropy in the mixture compared to pure, or nearly pure, DIPA. This is direct experimental support for molecular clustering as the source of the negative entropy of mixing that underlies the important LCST phenomenon and yields novel microscopic insights into the origins of the phenomenon.