Synagogue teacher's endless talk (5)
I believe the answer is:
rabbi
'synagogue teacher's' is the definition.
(I've seen this in another clue)
'endless talk' is the wordplay.
'endless' means to remove the last letter.
'talk' becomes 'rabbit' (rabbit can informally mean to talk).
'rabbit' with its last letter taken off is 'RABBI'.
(Other definitions for rabbi that I've seen before include "Minister of a synagogue" , "Sacks, for example" , "Hebrew teacher" , "Religious scholar" , "Blue" .)