Privately, rabbis hopeful about senior member of the Christian clergy (6)
I believe the answer is:
bishop
'senior member of the christian clergy' is the definition.
(I've seen this before)
'privately rabbis hopeful about' is the wordplay.
'about' indicates a hidden word (the clue goes around or about the answer).
'BISHOP' is hidden within 'privately rabbis hopeful'.
(Other definitions for bishop that I've seen before include "Senior church minister" , "Senior churchman" , "Powerful cleric" , "Head of a diocese" , "High-ranking clergyman" .)