Crossword Genius

Ditch local environmentalist that's no good (4)

Ross

I believe the answer is:

reen

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

'ditch' is the definition.
(I know that reen is a type of ditch)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for reen that I've seen before include "Ditch" , "West Country watercourse" .)

I've seen this clue in The Times and The Sunday Times.
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