Person taking action against senior officer is powerless (4)

I believe the answer is:

suer

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

'person' is the definition.
Both the answer and definition are singular nouns.
Maybe they are linked in a way I don't understand?

I don't understand the rest of the clue.

Can you help me to learn more?

(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.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.