Important sort of number (5)
I believe the answer is:
prime
'sort of number' is the definition.
(I know that prime is a type of number)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for prime that I've seen before include "Best - get ready" , "Chief or most important, like a minister" , "Chief or best quality" , "Main; make ready" , "put in the picture" .)