Crossword Genius

Rough passage, it's said? (6)

Ross

I believe the answer is:

coarse

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'said?' is the definition.
Both the definition and answer are adjectives. Perhaps you can see a link between them that I don't see?

'rough passage' is the wordplay.
'rough' becomes 'co' (I can't explain this - if you can you should believe this answer much more).
'passage' becomes 'arse' (arse is a kind of passage).
'co'+'arse'='COARSE'

'it's' acts as a link.

Can you help me to learn more?

(Other definitions for coarse that I've seen before include "Unpolished -- ribald" , "Rought or crude" , "gross" , "lacking in taste" , "Rough in texture or crude in manner" .)

I've seen this clue in The Telegraph.
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