Fail to be a turncoat, we hear? It’s intrusive (9)

I believe the answer is:

knotgrass

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'fail' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for knotgrass that I've seen before include "Weed" , "plant" .)

I've seen this clue in The Times.

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