The mysteries of opening artery on Royal Canal, for the most part (6)
I believe the answer is:
arcana
'the mysteries' is the definition.
(I've seen this in another clue)
'opening artery on royal canal for the most part' is the wordplay.
'opening' says to take the initial letters.
'on' says to put letters next to each other.
'royal' becomes 'r' (abbreviation).
'for the most part' means to remove the last letter.
The initial letter of 'artery' is 'a'.
'canal' with its final letter taken off is 'cana'.
'a'+'r'+'cana'='ARCANA'
'of' acts as a link.
(Other definitions for arcana that I've seen before include "Secret things" , "Secrets, mysteries" , "Tarot cards" .)