Crossword Genius

A silly place to grow plants? (6,3)

Ross

I believe the answer is:

window box

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

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

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for window box that I've seen before include "where flowers may be found" , "Plant container on sill" , "Narrow container for plants" .)

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