Cleric who again takes the doctor's part (6)
I believe the answer is:
rector
'cleric who again' is the definition.
'rector' can be an answer for 'cleric' (type of clergyman). I'm not sure about the remainder of the definition.
'takes the doctor's part' is the wordplay.
'takes' becomes 'rr' (abbreviation for recipe - 'take' in Latin - twice).
'the doctor's' becomes 'ecto' (I can't justify this - if you can you should give a lot more credence to this answer).
'part' is an insertion indicator (the inserted letters split open or part the word).
'rr' placed around 'ecto' is 'RECTOR'.
Can you help me to learn more?
(Other definitions for rector that I've seen before include "Type of clergyman" , "Head of some colleges" , "Member of the clergy" , "Minister of religion" , "Parish clergyman" .)