Talk at length trivially, for the most part, to religious teacher (5)
I believe the answer is:
rabbi
'to religious teacher' is the definition.
(I've seen this before)
'talk at length' is the wordplay.
'talk' becomes 'abbi' (I can't justify this - if you can you should believe this answer much more).
'at' says to put letters next to each other (I've seen this in other clues).
'length' becomes 'r' (r is a kind of length).
'abbi' after 'r' is 'RABBI'.
'trivially for the most part' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
(Other definitions for rabbi that I've seen before include "Religious scholar" , "A Jewish clergyman" , "Sacks, for example" , "Jewish religious teacher" , "Religious person" .)