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They don't make a prison -- delays

Ross

I believe the answer is:

stonewalls

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'prison delays' is the definition.
Although both the answer and definition are verbs in their -s form, I don't understand how one could define the other.

'they don't make a' is the wordplay.
I cannot quite understand how this works, but
'a' could be 'one' and 'one' is found in the answer.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for stonewalls that I've seen before include "Obstructs progress" , "Goes slow deliberately" .)

I've seen this clue in The Telegraph.
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