Royal Mile, say (8)
I believe the answer is:
imperial
This clue might be a double definition.
'royal' is the first definition.
(synonyms)
'mile say' is the second definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
Can you help me to learn more?
(Other definitions for imperial that I've seen before include "Supreme, august; a beard" , "Majestic - not metric" , "Like some rulers" , "Relating to empire" , "Magnificent; mint --" .)