Teacher to talk endlessly (5)
I believe the answer is:
rabbi
'teacher' is the definition.
(Jewish teacher)
'talk endlessly' is the wordplay.
'talk' becomes 'rabbit' (rabbit can informally mean to talk).
'endlessly' means to remove the last letter.
'rabbit' with its last letter taken away is 'RABBI'.
'to' acts as a link.
(Other definitions for rabbi that I've seen before include "Religious person" , "Hebrew teacher" , "Jewish learned man" , "Leader of worshippers" , "Hebrew scholar" .)