Crossword Genius

They're used by scriptwriters (7)

Ross

I believe the answer is:

letters

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'scriptwriters' is the definition.
Although both the answer and definition are plural nouns, I can't understand how one could define the other.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for letters that I've seen before include "They are bound to look after tenants" , "Written notes" , "They may indicate a degree" , "Literary culture" , "A, B, C, D etc" .)

I've seen this clue in the Evening Standard.
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