Territory for 150 in Eastern part of church (7)
I believe the answer is:
enclave
'territory' is the definition.
(enclave is a kind of territory)
'150 in eastern part of church' is the wordplay.
'150' becomes 'cl' (roman numerals).
'in' is an insertion indicator.
'eastern' becomes 'e'.
'part of church' becomes 'nave' ('nave' is part of 'church').
'e'+'nave'='enave'
'cl' inserted into 'enave' is 'ENCLAVE'.
'for' acts as a link.
(Other definitions for enclave that I've seen before include "Area enclosed within a foreign territory" , "Region wholly within another" , "Land surrounded by another country" , "Part of country surrounded by foreign territory" , "Small territory owned by one state but lying within another" .)