Rouen's top priest there holding large riotous mass (6)
I believe the answer is:
rabble
'riotous mass' is the definition.
'rabble' can be an answer for 'mass' (thesaurus). I'm unsure of the 'riotous' bit.
'rouen's top priest there holding large' is the wordplay.
'top' indicates taking the first letters (in a down clue, the first letter is top).
'priest there' becomes 'abbe' (I can't justify this - if you can you should give a lot more credence to this answer).
'holding' indicates putting letters inside.
'large' becomes 'L' (eg in clothes sizes).
The first letter of 'rouen' is 'r'.
'abbe' going around 'l' is 'abble'.
'r'+'abble'='RABBLE'
Can you help me to learn more?
(Other definitions for rabble that I've seen before include "Unruly mob" , "Hoi polloi" , "Person manipulating mob's passion" , "Disorderly, loud crowd" , "A disorderly crowd of unruly people" .)