The field of Gray's Elegy (3)

I believe the answer is:

lea

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'the field' is the definition.
(lea can mean a meadow or field of grass)

I don't understand the remainder of the clue.

(Other definitions for lea that I've seen before include "River of east London" , "Land growing grass" , "River flowing through London" , "area of grassland" , "'The lowing herd wind slowly o'er the . . .'" .)

I've seen this clue in the Evening Standard.

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