Crossword Genius

Some grasses become hostile if caught (4)

Ross

I believe the answer is:

ryes

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

'some grasses' is the definition.
(I've seen this in another clue)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Another definition for ryes that I've seen is " cereals".)

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