Priest, senior religious figure not using Old Testament in scripture lessons (5)
I believe the answer is:
rabbi
'priest' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for rabbi that I've seen before include "Minister of a synagogue" , "Jewish religious teacher" , "A Jewish priest" , "Jewish minister" , "Hebrew scholar" .)