Relish the "Don't be such a slowcoach!" (7)

I believe the answer is:

catchup

I'm a little stuck... Click here to teach me more about this clue!

This clue could be a double definition.

'relish the don't be' is the first definition.
The definition suggests a singular noun which matches the answer.

'a slowcoach' is the second definition.
Both the definition and answer are singular nouns.
Maybe they are linked in a way I don't understand?

'such' is the link.

Can you help me to learn more?

(Another definition for catchup that I've seen is " Sauce!".)

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