The Vatican's one English hub accommodates 150 (7)
I believe the answer is:
enclave
'the vatican's one' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'english hub accommodates 150' is the wordplay.
'english' becomes 'e' (abbreviation).
'hub' becomes 'nave' (I am not sure about this - if you are sure you should believe this answer much more).
'accommodates' means one lot of letters goes inside another.
'n50' becomes 'cl' (roman numerals).
'nave' placed around 'cl' is 'nclave'.
'e'+'nclave'='ENCLAVE'
Can you help me to learn more?
(Other definitions for enclave that I've seen before include "Enclosed piece of territory" , "Area ethnically different from its surroundings" , "Vatican City, say" , "Enclosed territory, culturally distinct the one that surrounds it" , "Land surrounded by another country" .)