This paper explores the possibility of entities dynamically identifying and subleasing licensed spectrum for short time periods, from other entities that may own spectrum. For example, mobile operators often need more spectrum than available during peak demand, while parts of the available spectrum may go underutilized at other times when network load is low. We seek to enable dynamic subleasing of the underutilized spectrum in this situation. To that end, we propose Spectraleas: a system that helps spectrum owners (operators) identify available (needed) spectrum, then matches operators and owners with short-term spectrum needs/availability and initiates co-agreed leases through a spectrum marketplace architecture. While exclusively licensed spectrum will remain the preferred answer for mobile operators, Spectraleas enables spectrum owners to lease spectrum to other entities as needed while maintaining full control over when and where such spectrum is leased. We construct a model of utilized spectrum to take advantage of time varying demands across spectrum owners to allow operators with temporary spectral needs to utilize under-utilized spectrum that other spectrum owners may possess. We identify key challenges that need to be addressed and engineer an architecture to enable such dynamic and temporary sharing of spectrum, down to the level of an individual cell. By analyzing real, active measurements on commercial mobile networks, we find opportunities for spectrum subleasing and show the feasibility of our approach by implementing a prototype mimicking two test operators over a university campus wireless testbed.