Something like skilly, taken as punishment (5)

I believe the answer is:

gruel

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'punishment' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for gruel that I've seen before include "sloppy fare" , "Unappetising food" , "Thin soup" , "Luger (anag)" , "cheap fare" .)

I've seen this clue in the Evening Standard.

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