The Collaborative International Dictionary of English v.0.48:

Exclave \Ex*clave"\, n. [Formed fr. enclave by substitution of
   ex- for en-]
   A portion of a country which is separated from the main part
   and surrounded by politically alien territory. [Recent.]
   [1913 Webster]

   Note: The same territory is an enclave in respect to the
         surrounding country and an exclave with respect to the
         country to which it is politically attached.
         [1913 Webster]