Cherished royal insider is pretty dull (5)
I believe the answer is:
drear
'dull' is the definition.
(I've seen this before)
'cherished royal insider is pretty' is the wordplay.
I cannot really see how this works, but
'royal' could be 'r' (abbreviation) and 'r' is located in the answer.
'cherished' could be 'dear' (dear can mean cherished or adored) and 'dear' is present in the leftover letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for drear that I've seen before include "Morose" , "Poetic dull and uninteresting" , "Dismal, dull" , "Gloomy and cheerless" , "Grim" .)