The Collaborative International Dictionary of English v.0.48:

Undertenant \Un"der*ten`ant\, n.
   The tenant of a tenant; one who holds lands or tenements of a
   tenant or lessee.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

UNDER-TENANT. One who holds by virtue of an underlease. (q.v.) See
Subtenant.