Crossword Genius

One opposed to prosecution? (7)

Ross

I believe the answer is:

defence

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'prosecution?' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Maybe you can see a link between them that I can't see?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for defence that I've seen before include "shelter" , "Case for the accused" , "See 12" , "This department deals with national protection" , "Protection; justification" .)

I've seen this clue in the Evening Standard.
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