If it runs, it's not particularly fast (3)
I believe the answer is:
dye
'fast' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for dye that I've seen before include "Soluble substance for staining or colouring" , "Colouring matter" , "Pigment" , "Staining material" , "Colour by dipping in liquid" .)