Person taking action against senior officer is powerless (4)

I believe the answer is:

suer

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'person' is the definition.
Both the definition and answer are singular nouns.
Perhaps you can see a link between them that I can't see?

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

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(Other definitions for suer that I've seen before include "One seeking redress" , "person in court" , "One in court, perhaps" .)

I've seen this clue in The Times.

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