Is it ensure, insure, or assure?
1868, from French enclave, from Old French enclaver "enclose, comprise, include" (13c.), from Late Latin inclavare "shut in, lock up," from Latin in- "in" (see in- (2)) + clavis "key" (see slot (n.2)). Enclaved "surrounded by land owned by another" is attested in English from mid-15c., from Middle French enclaver.
enclave en·clave (ěn'klāv', ŏn'-)
A detached mass of tissue enclosed in tissue of another kind.