Prisoners badmouth ugliest goon (4)

I believe the answer is:

thug

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

'goon' is the definition.
(I know that goon can be written as thug)

'prisoners badmouth ugliest' is the wordplay.
'THUG' is hidden within the letters.
But, I am unsure how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for thug that I've seen before include "Indian strangler - criminal" , "Gangster" , "Hooligan" , "Bully" , "Brutal criminal" .)

I've seen this clue in The Irish Times.

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