Crossword Genius

The effect they have on cars is baffling (9)

Ross

I believe the answer is:

silencers

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

'the effect they' is the definition.
The definition suggests a plural noun which matches the answer.

I can't explain the remainder of the clue.

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I've seen this clue in the Evening Standard.
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