Definitions of underlease
a lease granted by a tenant or lessee; a lease granted by a tenant or lessee at less than the usual value. A lease is the agreement that gives a person or business entity (the tenant) the right to possess or occupy property either for life, for a fixed period of time or for a period that may be ended at the will of one or both parties.
Party B agrees not to underlease, sublease, or transfer the leased premises without the written agreement of Party A.