Main sort of number (5)
I believe the answer is:
prime
I believe this clue is a double definition.
'main' is the first definition.
(thesaurus)
'sort of number' is the second definition.
(I know that prime is a type of number)
(Other definitions for prime that I've seen before include "Chief or most important, like a minister" , "Number with no factor but itself and 1" , "Main; make usable" , "At least two" , "Prepare to fire - first" .)