Splendid gem (9)
I believe the answer is:
brilliant
This clue is a double definition.
'splendid' is the first definition.
(I know that splendid can be written as brilliant)
'gem' is the second definition.
(I know that brilliant is a type of gem)
(Other definitions for brilliant that I've seen before include "Exceptionally clever" , "Shining intensly" , "Excellent; a diamond" , "'Wonderful, dazzling (9)'" , "Great" .)