Language of the East used in sacred book (6)
I believe the answer is:
korean
'language' is the definition.
(Korean is an example)
'the east used in sacred book' is the wordplay.
'the east' becomes 'e' ('E' can be a synonym of 'east').
'used in' means one lot of letters goes inside another.
'sacred book' becomes 'koran' (I've seen this before).
'e' inserted inside 'koran' is 'KOREAN'.
'of' is the link.
(Other definitions for korean that I've seen before include "Asian language" , "Citizen of divided country" , "Asian national" , "Eg, from Seoul" , "From Pyongyang, perhaps" .)