A heartless religious teacher with cold language (6)
I believe the answer is:
arabic
'language' is the definition.
(Arabic is a language)
'a heartless religious teacher with cold' is the wordplay.
'heartless' says to hollow out the word (remove centre letters).
'religious teacher' becomes 'rabbi' (Jewish teacher).
'with' means one lot of letters go next to another.
'cold' becomes 'C' (eg on taps).
'rabbi' with its centre removed is 'rabi'.
'a'+'rabi'+'c'='ARABIC'
(Other definitions for arabic that I've seen before include "There's a CIA bar on this language, oddly" , "Gum --, ingredient of glue and incense" , "Language of North Africa and the Middle East" , "Major language of Middle East" , "Language (of numbers)" .)