Do they hold the prison service together? (6)

I believe the answer is:

screws

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'do they' is the definition.
The definition suggests a plural noun which matches the answer.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for screws that I've seen before include "Prison warders (sl.)" , "Fix in place (6)" , "Put on" , "Fastner with spiral thread" , "Cheats" .)

I've seen this clue in the Evening Standard.

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