Is it possible out-of-head Gary will get grass? (6)

I believe the answer is:

canary

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

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

I don't understand the rest of the clue.

(Other definitions for canary that I've seen before include "Bird from an island group" , "Songbird once used to detect toxic gases in mines" , "Kind of yellow" , "Bright yellow finch" , "Caged song bird" .)

I've seen this clue in The Independent.

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