They may be fast, but only without running (4)

I believe the answer is:

dyes

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'they' is the definition.
Both the answer and definition are plural nouns.
Maybe there's a link between them I don't understand?

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for dyes that I've seen before include "Colours (fabric)" , "Stains" , "Colour" , "Colouring materials" , "Substances for staining or colouring" .)

I've seen this clue in The Irish News.

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