Crossword Genius

Clean river? (3)

Ross

I believe the answer is:

lea

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

'river?' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

'clean' is the wordplay.
'LEA' is hidden within the letters.
However, I am unsure how this is indicated.

Can you help me to learn more?

(Other definitions for lea that I've seen before include "Open area of grassy or arable land" , "Open pasture" , "Grazing land" , "Poetically, a piece of meadow or pasture" , "Meadow (poetic)" .)

I've seen this clue in the Evening Standard.
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