Crossword Genius

Person with whom the prosecution opens? (3,7)

Ross

I believe the answer is:

key witness

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

'whom the prosecution opens?' is the definition.
I can't tell whether this defines the answer.

'person' is the wordplay.
I cannot really understand how this works, but
'person' could be 'witness' (witness is a kind of person) and 'witness' is found within the answer.
The remaining letters 'key' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

'with' is the link.

Can you help me to learn more?

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