Red number (8)
I believe the answer is:
cardinal
This clue is a double definition.
'red' is the first definition.
(I know that cardinal is a type of red)
'number' is the second definition.
(I know that cardinal is a type of number)
(Other definitions for cardinal that I've seen before include "of the highest importance" , "Main - church dignitary" , "Key" , "of prime importance" , "Vivid red of a senior bishop?" .)