Grass in one of the Cinque Ports (3)

I believe the answer is:

rye

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'grass' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

(Other definitions for rye that I've seen before include "Hardy cereal crop" , "Gypsy gentleman" , "East Sussex resort" , "old port" , "Wheat-like cereal plant" .)

I've seen this clue in The Guardian.

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