A line in the field of agriculture (6)

I believe the answer is:

furrow

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'a line' is the definition.
(I know that line can be written as furrow)

I don't understand the remainder of the clue.

(Other definitions for furrow that I've seen before include "It's groovy!" , "Wrinkle (brow)" , "'Rut, groove (6)'" , "Groove or trench" , "Long shallow trench in the ground" .)

I've seen this clue in the Evening Standard.

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