Grass snake in enclosed space might cause this? (5)

I believe the answer is:

panic

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

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

I can't explain the remainder of the clue.

(Other definitions for panic that I've seen before include "Fear out of sixteen ac" , "Flap" , "Greek god" , "Sudden uncontrollable fear" , "Great consternation" .)

I've seen this clue in The Times.

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