Is it instrumental in warning Parisian pedestrians? (6,4)

I believe the answer is:

french horn

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'instrumental in warning parisian pedestrians?' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for french horn that I've seen before include "Brass musical instrument" , "Wind instrument" , "part of a broken-down 27 [CARTHORSE]?" .)

I've seen this clue in The Globe and Mail.

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