About the drawback to being a guard (6)

I believe the answer is:

warder

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'a guard' is the definition.
(thesaurus)

I don't understand how the rest of the clue works.

(Other definitions for warder that I've seen before include "Gaoler" , "Prison officer" , "screw" , "Jailer" , "One is in charge of prisoners" .)

I've seen this clue in the Evening Standard.

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