Your informant at the zoo? (5,5)

I believe the answer is:

grass snake

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

'your' is the definition.
I can't tell whether this definition defines the answer.

'informant at the zoo?' is the wordplay.
'informant' becomes 'grass' (term for an informant or traitor).
'at the zoo?' becomes 'snake' (I can't explain this - if you can you should believe this answer much more).
'grass'+'snake'='GRASS SNAKE'

Can you help me to learn more?

(Another definition for grass snake that I've seen is " legless wild thing".)

I've seen this clue in the Evening Standard.

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