The kind of snake to betray one to the police? (5)

I believe the answer is:

grass

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'the kind of snake' is the definition.

I don't understand the remainder of the clue.

(Other definitions for grass that I've seen before include "Turf" , "Narrow-leaved green herbage" , "drugs" , "What meadowlark perches on" , "Eg, bent or fescue" .)

I've seen this clue in the Evening Standard.

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