Don't dash as much as one out on first delivery (7)

I believe the answer is:

runless

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'don't' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see a link between them that I can't see?

'dash as much as one out on first delivery' is the wordplay.
'dash' becomes 'run' (I've seen this before).
'as much as one out on first delivery' becomes 'less' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'run'+'less'='RUNLESS'

Can you help me to learn more?

I've seen this clue in The Times.

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