Start asking reader about book in Celtic language (6)
I believe the answer is:
arabic
'language' is the definition.
(Arabic is a language)
'start asking reader about book in celtic' is the wordplay.
'start' says to take the initial letters.
'celtic' becomes 'C'.
The initial letters of 'asking reader about book in' is 'arabi'.
'arabi'+'c'='ARABIC'
(Other definitions for arabic that I've seen before include "There's a CIA bar on this language, oddly" , "Language of North Africa and the Middle East" , "Major language of Middle East" , "foreign language" , "Language of the Koran" .)